Because I have learned most everything that I know about web design on a trial and error basis.... I started out with mainly print design, it took me a while to start understanding web file types and what they are for. So here goes my opinion (like I said, I am self taught, so don't take this as the "Design Standard"):
GIF vs. PNG
GIF and PNG files are for web, and ONLY web! Now, at first my confusion with the files was, what is the difference between a gif and a png.... They have some similarities. They both allow you to save with a transparent background. What I have found to be the greatest difference between the 2 is the quality, and size. If you are going for high quality, I use PNG, but if you need a smaller file size, go with GIF. So if I am saving a Header or Button that needs to look really nice, and and have a good clean transparent background, I would go for the PNG, but if I was using some thing very large, like a background, I would go for the GIF (that way, it will load quicker for your viewers).
JPEG
JPEG is really pretty simple..... photos.
**If you have other insights, please comment! I love learning and I know that there is a lot more to file types that is worth mentioning.
Relax and enjoy, don't be rush.
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