diff --git a/appservice/account.go b/appservice/account.go
index 2df2930..3791ee3 100644
--- a/appservice/account.go
+++ b/appservice/account.go
@@ -137,9 +137,13 @@ func (a *Account) userInfoUpdatedInternal(user UserID, info *UserInfo) error {
 	}
 
 	if info.Avatar != nil {
-		err2 := mx.ProfileAvatar(mx_user_id, info.Avatar)
-		if err2 != nil {
-			err = err2
+		cache_key := fmt.Sprintf("%s/user_avatar/%s", a.Protocol, user)
+		cache_val := info.Avatar.Filename()
+		if cache_val == "" || dbCacheTestAndSet(cache_key, cache_val) {
+			err2 := mx.ProfileAvatar(mx_user_id, info.Avatar)
+			if err2 != nil {
+				err = err2
+			}
 		}
 	}
 
@@ -185,9 +189,13 @@ func (a *Account) roomInfoUpdatedInternal(roomId RoomID, author UserID, info *Ro
 	}
 
 	if info.Picture != nil {
-		err2 := mx.RoomAvatarAs(mx_room_id, info.Picture, as_mxid)
-		if err2 != nil {
-			err = err2
+		cache_key := fmt.Sprintf("%s/room_picture/%s", a.Protocol, roomId)
+		cache_val := info.Picture.Filename()
+		if cache_val == "" || dbCacheTestAndSet(cache_key, cache_val) {
+			err2 := mx.RoomAvatarAs(mx_room_id, info.Picture, as_mxid)
+			if err2 != nil {
+				err = err2
+			}
 		}
 	}
 
@@ -204,6 +212,9 @@ func (a *Account) Event(event *Event) {
 }
 
 func (a *Account) eventInternal(event *Event) error {
+	// TODO: automatically ignore events that come from one of our bridged matrix users
+	// TODO: deduplicate events if we have several matrix users joined the same room (hard problem)
+
 	mx_user_id, err := dbGetMxUser(a.Protocol, event.Author)
 	if err != nil {
 		return err
diff --git a/appservice/db.go b/appservice/db.go
index 5cefd30..646dae3 100644
--- a/appservice/db.go
+++ b/appservice/db.go
@@ -23,6 +23,8 @@ func InitDb() error {
 		return err
 	}
 
+	db.AutoMigrate(&DbCache{})
+
 	db.AutoMigrate(&DbUserMap{})
 	db.Model(&DbUserMap{}).AddIndex("idx_protocol_user", "protocol", "user_id")
 
@@ -35,6 +37,14 @@ func InitDb() error {
 	return nil
 }
 
+// Long-term cache entries
+type DbCache struct {
+	gorm.Model
+
+	Key string `gorm:"unique_index"`
+	Value string
+}
+
 // User mapping between protocol user IDs and puppeted matrix ids
 type DbUserMap struct {
 	gorm.Model
@@ -76,6 +86,30 @@ type DbPmRoomMap struct {
 
 // ----
 
+func dbCacheGet(key string) string {
+	var entry DbCache
+	if db.Where(&DbCache{Key: key}).First(&entry).RecordNotFound() {
+		return ""
+	} else {
+		return entry.Value
+	}
+}
+
+func dbCachePut(key string, value string) {
+	var entry DbCache
+	db.Where(&DbCache{Key: key}).Assign(&DbCache{Value: value}).FirstOrCreate(&entry)
+}
+
+func dbCacheTestAndSet(key string, value string) bool {
+	// TODO make this really an atomic operation
+	// True if value was changed, false if was already set
+	if dbCacheGet(key) != value {
+		dbCachePut(key, value)
+		return true
+	}
+	return false
+}
+
 func dbGetMxRoom(protocol string, roomId connector.RoomID) (string, error) {
 	var room DbRoomMap
 
diff --git a/connector/mattermost/mattermost.go b/connector/mattermost/mattermost.go
index 73ea66b..6282831 100644
--- a/connector/mattermost/mattermost.go
+++ b/connector/mattermost/mattermost.go
@@ -314,17 +314,21 @@ func (mm *Mattermost) handleConnected() {
 				} else {
 					room_info.Name = t.Team.Name + " / " + room_info.Name
 				}
-				// TODO: cache last update time so we don't do this needlessly
 				if t.Team.LastTeamIconUpdate > 0 {
-					team_img, resp := mm.conn.Client.GetTeamIcon(t.Id, "")
-					if resp.Error == nil {
-						room_info.Picture = &BlobMediaObject{
-							ObjectFilename: t.Team.Name,
-							ObjectMimetype: http.DetectContentType(team_img),
-							ObjectData: team_img,
-						}
-					} else {
-						log.Warnf("Could not get team image: %s", resp.Error)
+					room_info.Picture = &LazyBlobMediaObject{
+						ObjectFilename: fmt.Sprintf("%s-%d",
+							t.Team.Name,
+							t.Team.LastTeamIconUpdate),
+						GetFn: func(o *LazyBlobMediaObject) error {
+							team_img, resp := mm.conn.Client.GetTeamIcon(t.Id, "")
+							if resp.Error == nil {
+								log.Warnf("Could not get team image: %s", resp.Error)
+								return resp.Error
+							}
+							o.ObjectData = team_img
+							o.ObjectMimetype = http.DetectContentType(team_img)
+							return nil
+						},
 					}
 				}
 				break
@@ -378,20 +382,24 @@ func (mm *Mattermost) updateUserInfo(user *model.User) {
 			DisplayName: userDisp,
 		}
 		if user.LastPictureUpdate > 0 {
-			// TODO: cache last update time so we don't do this needlessly
-			img, resp := mm.conn.Client.GetProfileImage(user.Id, "")
-			if resp.Error == nil {
-				ui.Avatar = &BlobMediaObject{
-					ObjectFilename: user.Username,
-					ObjectMimetype: http.DetectContentType(img),
-					ObjectData: img,
-				}
-			} else {
-				log.Warnf("Could not get profile picture: %s", resp.Error)
+			ui.Avatar = &LazyBlobMediaObject{
+				ObjectFilename: fmt.Sprintf("%s-%d",
+					user.Username,
+					user.LastPictureUpdate),
+				GetFn: func(o *LazyBlobMediaObject) error {
+					img, resp := mm.conn.Client.GetProfileImage(user.Id, "")
+					if resp.Error == nil {
+						log.Warnf("Could not get profile picture: %s", resp.Error)
+						return resp.Error
+					}
+					o.ObjectData = img
+					o.ObjectMimetype = http.DetectContentType(img)
+					return nil
+				},
 			}
-			mm.handler.UserInfoUpdated(userId, ui)
-			mm.userdisplaynamemap[userId] = userDisp
 		}
+		mm.handler.UserInfoUpdated(userId, ui)
+		mm.userdisplaynamemap[userId] = userDisp
 	}
 }
 
diff --git a/connector/mediaobject.go b/connector/mediaobject.go
index a8d6f9a..244e571 100644
--- a/connector/mediaobject.go
+++ b/connector/mediaobject.go
@@ -132,3 +132,53 @@ type nullCloseReader struct {
 func (ncr nullCloseReader) Close() error {
 	return nil
 }
+
+// ----
+
+type LazyBlobMediaObject struct {
+	ObjectFilename string
+	ObjectMimetype string
+	ObjectImageSize *ImageSize
+	ObjectData []byte
+
+	GetFn func(o *LazyBlobMediaObject) error
+}
+
+func (m *LazyBlobMediaObject) Filename() string {
+	return m.ObjectFilename
+}
+
+func (m *LazyBlobMediaObject) Size() int64 {
+	if m.ObjectData == nil {
+		m.GetFn(m)
+	}
+	return int64(len(m.ObjectData))
+}
+
+func (m *LazyBlobMediaObject) Mimetype() string {
+	if m.ObjectData == nil {
+		m.GetFn(m)
+	}
+	return m.ObjectMimetype
+}
+
+func (m *LazyBlobMediaObject) ImageSize() *ImageSize {
+	if m.ObjectData == nil {
+		m.GetFn(m)
+	}
+	return m.ObjectImageSize
+}
+
+func (m *LazyBlobMediaObject) Read() (io.ReadCloser, error) {
+	if m.ObjectData == nil {
+		err := m.GetFn(m)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return nullCloseReader{bytes.NewBuffer(m.ObjectData)}, nil
+}
+
+func (m *LazyBlobMediaObject) URL() string {
+	return ""
+}