>390x677
>517x812

Yeah, make them smaller, mister fucking guy.

Yup. There's no need for anything wider than 200px, and even that is wasteful. Aim for 150px wide and you should be fine.

.png is usually good for stuff with text (like our logo and banner) but it's also a larger filesize. .jpg is great for images without words, as it's a compressed format and is designed for use on the web to save space. Unless you're using an animated avatar, never use .gif. The alpha channel doesn't support partial transparency (either all transparent or no transparent per pixel) and it tends to be a clunky filesize.