if youre only planning on posting your art online, them please save it as .png ;this is also better for transparencies as well

BUT

please, if youre planning of printing your art, NEVER use png. it makes the quality of the image pretty shitty. use jpeg or pdf instead. and always set your work at 300dpi to get a better printing quality - this means, the images are crisper and sharper and theres no slight blurriness. i had a talk with my friend who is currently taking design, and pdf is much better to use when youre working with a bigger publishing company because it still has the layers intact, but if youre only planning on printing your stuff at staples or at some small publishing store, the jpeg is the way to go.