Blogstorm website has a useful wordpress plugin that helps you to build backlinks from your blog images.
Hotlink Builder plugin targets visitors that visit your blog fom image search engines and try to Hotlink your blog images.
Whenever a visitor enters your blog through image search engines like Google Images, Yahoo Images or MSN Live Search Images the script gets activated.
The script detects when the hotlinker right clicks on your blog image and brings up a text box with html code to insert the image in their blog or website with an appropriate backlink to your website.
If you have good number of images indexed in image search engines and if a large number of images are being hotlinked, you must give this plugin a try.
The plugin and installation instructions can be found on this page. The plugin is also available as a php script to be used on normal php websites, get the script and usage instructions from here.
You can also see this screencast to know how this plugin works in practice.