Know your audience. If your audience is beginning radiologists or students, focus the discussion on basics. If your cases are aimed at practicing radiologists, different content is required. Web images are not to be used for a final radiology report. Therefore, the images need be only large enough to clearly demonstrate the finding of interest.
Thumbnail images are a waste of time.
Inline images are the most efficient way to display images and are preferable to placing just links to the images. JPEG images usually are smaller and take less time to download than GIF images. In general, image quality is not substantially different, but download times are. Do not include more images than necessary to make the teaching point. A teaching file page should ideally consist of one or two, and no more than four, images.