I added buttons to the menu in tf2 with custom images, cs:s should be similar i think. I'm not 100% sure, but I think that you should use a .vtf (or .vmt) instead of a .tga
Не на 100% уверен, но думаю нужна картинка в формате .vtf (или .vmt). По Крайней мере так в tf2.