To add to Orwar's great explanation:
In the end, anything you upload to SL will become a JPEG and will lose some detail. The reason why you should save your work as PNG/TGA/PSD/etc is to keep the original quality on your computer as you made it.
The difference between PNG vs TGA is that PNG is losslessly compressed (to save space without sacrificing quality), while TGA is uncompressed. Realistically it doesn't matter, since they have the exact same capabilities.
I upload using TGA format, as I use masking in my work. Something to think about...if you use them.
