Blog image quality

I don't know why those images below look so fuzzy - I export them  at 200dpi, which I believe is better than screen resolution. I'm not sure if the problem is that that is an incompatible size, or if something happens to the image when I crop  (they automatically export from MS5 as A4 pages, and so with a large white border - cropping the border lets them appear bigge) and resize in my web editor.

They take long enough to export at 200dpi, I don't want to have to redo them. In the pat they have appeared fuzzy in my web editor, and then fine when view my published site on a web browser.