On the meta posts that collect community ads (example), I see ads in two sizes, 300x250px and 600x500px. (Everything renders the same when ads are actually used on the sites, as you would expect.) The requirements for ads include:
The image that you create must be 300 x 250 pixels, or double that if high DPI.
I assume "high DPI" is a property of the software used to create the ads, or perhaps some "image export" property. I'm not a graphics person; the ads I've made have usually started out as photos or similar art that I crop and scale to 300x250 using low-end software (Paintbrush on Mac, Paint on Windows, that kind of thing). Should I be making them double size? I haven't because nothing in my software says anything about "high DPI" so I assume it doesn't apply to me. In a few months we'll be doing the next round of ads on my sites, and if I should be doing something different to make better images, I'd like to know.