i use jpegs usually as the file size wont be super large while still having a good quality =o havent used png much..gifs when i have a transparent bg or a animation ^_^
gif when there is no great need for quality png when i need it to be optimum standard targa...... never
i use .png ~ for renders i make .jpg ~ for all of my other Photoshop art, save at highest detail, also highest file size(250GB HDD, no worries) .gif ~ for animations i make, like in an ava, but i dont really do these anymore .tga ~ for renders made in C4D, when i get back into 3d rendering (or maybe i use .tiff for the latter)