http://www.packtpub.com has a great book: "Learning eZ publish 3", and it has some supporting scripts.
One of the scripts they have allows you to upload content into the system - the filename starts with the word "create". You'll need to add this line into their scripts for compatibility with the new versions.
If you go to the publisher's site, you can go into the support area and download the scripts. You will have to give them your email (they send you a link).
I have not used it with images, but you may want to try. You should be very comfortable with PHP, and have shell access for this approach. I have used this script as the foundation for many uploads, some that were really complex.
I'd second Lukasz suggestion: with webdav, you can see your website as a regular network drive. Uploading images is then as simple as drag n' drop them to the proper location.