File types you should use: (jpg/jpeg, gif, and png)
The key thing to keep in mind about image file types is that whenever possible use JPG instead of PNG. Even if you create an image in PNG, save it as a JPG before using it on a website when it’s a larger graphic. You can always go back to the original png to make changes. PNGs can be multiple times the file size of the same image as a jpg. This is particularly important when doing large banner graphics.
If we used all PNG's in the Pub Site website portfolio and you go to one of the pages that have many screen captures on it, it will load slowly. We may not notice it much with our high-speed connections and new laptops, but not everyone has that and many people will be viewing on their phone, not on wifi.
JPGs have limitations though: they can't be transparent, and they don't have layers, which is why you have to keep the original png to go back and make edits.
Gifs can be transparent, but no layers. Not a lot of situations to use them.