This is a little trick I found out today:
1. Download the MC Launcher sense it doesn't work with browser which some new people sometimes use.
2. Drag it to where you want.
3. Right click it and make sure you have WinRAR because I think it's the best Archive
4. And there should be a option there that says "Open with WinRar"
5. Go to net then minecraft then you should see a .png file named favicon.
6. Change that to whatever picture you want. (Mines a Enderdragon :P)

And yes this is useless but it is indeed useful for your "Custom" needs.
(Such as wanting: Custom skins, Custom capes which Notch gives to special people, Custom Texturepacks, ect.)
PLEASE NOTE: Sometimes it won't work but trust me eventually it will work. (I add to try like 100 times XD)