Is it possible to only let K2 generate one single image size, instead of 3-4 different sizes?

And another thing, which file loads the components/com_k2/css/k2.css file?

Actually the first one is not possible, the css file is loaded in the helper files.
There is no need to edit anything since you can enable/ disable it from K2's params.

Hi Kasper,

I previously posted a solution that explains how to use Extra Fields to manage your item images:
www.joomlaworks.net/forum/k2-en/40376-solved-keeping-orginal-names-in-my-images#141480

I hope this helps,

Joe Campbell