Hi Leon,
I'm glad you mentioned the Image Compression option in Edit > Lesson Settings, because I was definitely going to suggest that first.
No, there is not a way to bulk replace the images in your sb/sb4 file - you will have to do that manually.
In the published output, all of the image files are in the
assets folder. So you could run a quick batch process using Photoshop or some other photo editor on all the files in that folder and replace them that way. As long as they have the same name/extension, you can swap those images out and the lesson won't know the difference. Then just re-zip the package and upload to the LMS (or however you're deploying the lesson).
Hope that helps! Let me know if anything is unclear.
- Nav