Fix some website image issues

Make sponsor logos not transparent where necessary

This avoids conflicts with black background from “dark mode” browser extensions. Two images are still missing, going by the prod website, but neither can be found here:

• https://www.teckids.org/theme/images/sponsors/proxmox.png
• https://www.teckids.org/theme/images/partners/osi.png

The bareos and fsfe logos are good.

gruppenbild.jpg not gruppenbild.jpeg

What it says. The .jpg one was already present and identical.