img alt attribute on user lists

can I modify the core files and just change the alt attribute to "firstname"?

I know its not the best solution but I dont know how to develop a plugin...

You can alter anything you want, but you void support. We do not support core edits and if we should have to review your install for whatever reason you will be required to remove core edits.

The below tutorial explains trigger usage and comes with a working example plugin so this could perhaps speed up development process of making your own plugin to do as you're wanting.

www.allmysocials.com/directory/tutorials/item/228-event-triggers

that tutorial looks very useful!

however, wasnt the problem that there is NO trigger that can be used before userlist display? Otherwise I could use the cb auto action system?

There are triggers before the display, but not after the display. Using PHP you should be able to parse the results and remove it in source or edit the avatar field all together and remove it in source.