You can change default picture in the registration by uploading new one here (remember to keep the same names for files):
http://your_site/components/com_community/templates/default/images/guest-bg.jpg
You can change default avatars by uploading new one to
http://your_site/components/com_community/assets/
And default image for events is here:
http://your_site/components/com_community/templates/default/images/cover/event-default.png
If you are using FireFox you always can check location of certain image by clicking right click on it and choosing "View Image Information"
or "View Background Image".
It's hanged:
prisma-schulen.de/mathelive/mathelive/co.../images/guest-bg.jpg
If you change images please try to refresh your web site with CTRL+F5 (if you are using FireFox). Old image is stored in browser cache so it need to be reloaded.
@Amanda: Please start your own topic I'll answer with details there :)
I suggest to do it by language overrides. I saw that you used it already :) Search for text e.g "Kostenlos registrieren!" and replace it with your own. Should work even after upgrades.
Topic closed.