thecoalman - 3:04 am on Jan 10, 2013 (gmt 0)

Your images should be the size of whatever real estate your taking up on the screen unless you're trying to provide higher resolution images for download.

PNG is lossless which by default makes it the better format but it also has larger file sizes. .jpg has sliding scale of quality, as you reduce quality you reduce file size. If you're only lightly compressing them you wont see any perceptible difference over .png Generally speaking .jpg is the better format for regular photo type images because you'll get good quality and smaller file sizes.

...many screens cannot render images at high dpi resolution.

DPI is meta data and irrelevant for web browsers. A 200px*200px image that is 72 dpi or 1000 dpi is the the exact same image and will render at the same exact size in a browser. It's not until you insert that image onto a canvas that has physical dimensions like a Word document that it becomes relevant