Is there any way to find broken images across an entire site?
You can try the linkchecker module.   It does check the source of &lt;img&gt; tags in the body, however it doesn't do it perfectly like it does for &lt;a&gt; tags.


If &lt;img&gt; have alt text, it thinks it is fine, even though the link is broken
&lt;img&gt; with links like "sites/testing/file/image.jpg" are returned not broken, links like "/sites/testing/file/image.jpg" are returned broken
External links work pretty well.


Your best bet is to use Xenu’s Link Sleuth project  (windows freeware)

Additional resources:
Here is a comparison of the link page modules.  Maybe you can find something else.
Broken Link Checker thread on drupal.org