Yes, images are what I am working on now.
In my old blog, which I am importing from, I stored most of my photos in Gallery2 and used the WPG2 plug-in to include them in WordPress. So now I have to migrate both Gallery2 and WordPress. I am actually migrating my Gallery2 albums into static HTML using sigal (
http://sigal.saimon.org) and will have to rewrite the wpg2 tags to standard img tags pointing to them.
For images that were uploaded to WordPress it seems like the simplest thing to do would be to copy the wp-content/uploads directory and setup your webserver to keep serving them from there. But do you want them in the Wagtail image library? I haven't got that far, but it seems like running a regular expression over each post to grab the filename, read it in to Wagtail might be a way to go.
All the best,
David.