Eagle_Owl
Now I can answer this question myself, because today my new try to upload an image was successful. :)
I have tried two pictures in several sizes:
a) 586x700 Pixel, 52 KB
b) 168x200 Pixel, 6 KB
c) 739x787 Pixel, 248 KB
All of them could be used as avatars today. This was not the case when the post was opened.
And the forum software has reduced it to 100x100 with 25 KB size.
Presumably an administrator has optimised the settings?
Many thanks for that!