Resolution Differences
15 years ago
Looking at the difference between http://www.furaffinity.net/view/4185098 and http://www.furaffinity.net/view/4187355 I realize I should probably start posting larger versions of my images. I tend to just bring everything down to 72dpi for web viewing by reflex. But that's probably kind of old school, now. Most people have much better internet connections and larger monitors from when I first started saving things for the web. Sometimes the smaller images can minimize some nice details.
FA+

I tend to post minimum 600x800, and on many occasions have only shrunk it down to fit the 1280x1280 size limits of the website.
Could probably go in the middle and do 150dpi or whatnot.
However, if you can trust that nobody will take your images, make personal prints and try to make money off of them, then put up higher quality. <3
Most screens now range between 90-110DPI, a good value nowadays is 96DPI (ratio of a 17",1280x1024 monitor)
All art posted on my Naorhy and Space Opera sites are 800x600x72 DPI. I don't care if people are using 13" monitors or 24" wide screen monitors. I have a duty to the artist that I purchase from not to give away high quality copies of images, images that might be found on a milkshake & crisps flyer in England one day. Yes, this happened to a FA Artist this year.
One of the FA features I've been campaigning for is for thumbnails to display their file size, so artists who routinely post gigantic images can lose traffic :). Dimension-wise, I hold mine to 750px on the shortest side. I never kept track of the actual DPI, other than knowing I work at 300, then resize to roughly 25% of that for display.
...And don't over-compensate with heavy JPEG compression, by the way. File sizes in the high-tens to low-one-hundreds aren't as important as they used to be.
Image quality is something about which I care a lot.
Remeber, resizing up is usually not possible with any reliable quality, but resizing down is easy, simply press the download button and most browsers will fit it to your window size for easy viewing. and lets not forget that for those with limited bandwidth or speed, there's always the preview view that resizes it down server side to send a smaller file.