It depends. Theoretically, the less images on a page the better, as your pages will load faster (and put less strain on your server).
There are two types of images. There are template-level images and post-level images. Template-level images exist in your blog’s header/footer/sidebar template, and therefore appear on every page on your site. Post-level images are part of your content, and they belong to an individual posts.
In your template, you should have as little images as possible. When you create a design, you want to keep the essential images to a minimum. Use tiles, well-optimized image blocks, etc. As of this writing, this blog’s design consists of two images (the logo and the tiled edge image). Once you have your mock-up of the design, figure out the best way to break it up. You want as little images as possible, and you want to keep them as small (as in kilobytes) as you can.
Also in your template you’ll have secondary images that server their own purposes, but aren’t part of the overall design. These can be RSS icons, 88×31 buttons, etc. You want to keep these to a minimum as well. You don’t have to be overly conservative, like some bloggers who are totally obsessed with design simplicity, but don’t go overboard either.
Post-level images are a different matter. It really depends on your post, the number of images. Don’t worry too much. After all, blogging is, first and foremost, about exchanging information. If it takes you a hundred images to get your point across, go ahead! After all, a picture is worth a thousand words. Therefore a hundred pictures equals 100,000 words, right? However, don’t go looking for a book deal yet…
The problem with having a lot of post-level images is the index and archive pages, and the huge number of images that will appear there. The solution? The <!–More–> tag.
Don’t worry about post-level images too much. Just optimize them correctly (in Photoshop, use the “Save for Web” dialog, not the ordinary “Save As”) and you’ll be alright.