Just go to dummyimage.com/640×480 and poof, an image to drag to your desktop and use in mockups.
Great roundup, including one I’ve never heard of but immediately want to try out, Sumo Paint.
I wonder if this could be integrated into a CMS?
Here’s a free Web-based tool that allows you to upload a large image and embed a widget to let your users view, zoom, and explore it. This is new to me, via @brianboyer.
Pull a random image from a category (or categories) and display it as a link to the post, with calls to the post title and other fields, with plenty of room for markup.