TGAs are a popular image format. They're not exclusive to Brawl. Look it up, you'll find a lot of applications that can open them.
I don't necessarily need help with CSPs by the way...the real problem is, with the current Cupash, if you reach a certain number of custom CSP's the game will crash upon loading the character select screen. I guess the repacking program doesn't work properly to compress the images in the way that the game will accept it after a certain number of files. That' just my guess, who knows what it really is..all I know is I pretty much gave up completing the character select screen 100% because the game will crash.
As for guides, you can look at this..
http://www.smashboards.com/showthread.php?t=228958
And just to clear up, you can already customize any character's CSP. The problem lies with having all of them customized TOGETHER. So far nobody has done it because of the freezing. The highest I get is about 170 portraits and any higher will cause it to crash the game. I know someone who has around 180'ish but he hasn't tried to go any further than that..due to laziness I guess. To be honest I've lost interest in doing it, maybe I'll finish it some day but I'm gonna have to speak to the creator of the repacking program first.