If I have a series of PNGs that I'd like to use as a single animated texture, would I be able to use them as they are, or would I first need to convert them to a different format, like a single TGA?
There are programs helping you in this task. I personally use Zwoptex. Basically you load all your pngs, it arranges them (translating and rotating each sprite) into a bigger image (you decide the size) called texture atlas. You can then export the atlas with a xml file containing the coordinates of each single sprite in the atlas.