Aw, thanks! OK, by the sound of it, you're making a CS mod, but even if you aren't, it doesn't matter. The idea is the same. Got a good graphics program? Look for a btns_main.bmp file in the directory of the game that you are modifying. It's in gfx/shell. Copy it into "The Portal's" gfx/shell folder. Now open up the copy you just pasted in with your graphics program (Even MS paint's fine for this!). Now change whatever you want...oh wait, I've told you the wrong info. Damn it! Sorry 'bout that, but I'm sure you can put what I just told you to good use. If you did what I said correctly in the tut., then when you change the mod's title in the liblist.gam, it should change automatically. If not, see if you can find a strings.lst file anywhere. There isn't one with hl, but I think there is with cs and opfor. Possibly bshift, but I don't think so. I can't find 'em anywhere though.
Hope this helps.