My web application allows users to upload images of items they are planning on selling. I'm trying to find a good compromise between having the images large enough and detailed enough for buyers to get an idea of the condition of the item and having the images small enough so that they don't slow down the site substantially.

In addition, I will also need thumbnails for a quick view of multiple items.

When a user uploads their file then, should I store both the original copy as well as a resized version on my server? If so, is there a general accepted limit on these file sizes that I should impose to get a good balance of quality and image size?