Clear cache from android settings using following steps:
1. Go to settings by swiping down from android home screen.
2. Look for 'apps' and find bbm under downloaded tab
3. Scroll down find clear cache.
4. Restart bbm/phone
5. If problem persist, tap force stop before clear cache. Then start bbm and see if it loads correctly.
Step #5 can be repeated around 3x until bbm loads chat window and friend's profile pics really fSt.
Not recommended leaving any group if you intend to rejoin afterward. Joining group may download old pictures thus further weighing down memory load.
