We all know using Bookmarks for this isn't really flexible, so I posted a photo url into a text widget. Look at my blog for the example: http://compartments.wordpress.com. I posted my photo in the sidebar using a text widget. Here's how:
1. Create a new post and upload a photo (see FAQs about uploading photos into your posts). Click on the "html" button on your post editing tool bar. Highlight the html between <img src="......"... /> and copy it. Save that post as a draft. Do not publish it.
2. Go to the "Presentation" tab and select "Sidebar Widgets." Move a "Text" widget over to the right hand column and open it. Paste the url that you just copied from the "Write Post" section. Write some text too, if you want. Then save it and view your site. Wallah!