Its not the resolution thats the problem, its the filesize. 500kb for a 800×500(or similar) jpeg is a high, especially for poor 56k users like me! :(
Considering you could optimise the jpeg quickly in photoshop to a smaller filesize without losing much of the quality. It’s just that some people may not bother waiting for the screenshot to load if it takes ages, and thus you lose out on people seeing your work.
Best of luck. :)