Unsolved How do I add icons to mods?

I'm new to mods and I don't know how to add icons...ㅠ.ㅠ

 

In the resource posting menu, at the bottom, there’s a checkmark for „Upload thumbnail image” or something like that. Then after you check that and after you press „Upload resource”, you get a window for choosing a thumbnail. Remember, it has to be 96x96 exactly.

 

Thank you!