Haven't really checked, whether others already postet a solution like this. Anyways... Accidentally I found a way to change group avatars and banners despite the fact of the buggy situation at the moment. But it works only in some cases, in some cases it doesn't.
It seems, that changing is only possible if your clan/group is not part of an alliance. And probably it's also not working if you are not the founder of the group.
1) go to your groups
2) choose the group, in which you like to have the changing
3) click on "Display Settings"
4) reload the page
5) click on "Display Settings" again
6) change your avatar and banner as you wish...
7) save changes
Hope it helps some of you, guys...