Hi @Craig_Ruaux,
You’re on the right track with supplementary files, I think. If you upload large versions of your images there, you can refer to them using specially-crafted URLs in your HTML galley:
ojs://suppfile/[article ID]/[supplementary file ID]
You can either code your HTML to get the browser to scale down that image for embedding in your HTML (quicker to do but not very network-efficient) or you can upload a thumbnail as an HTML image, linking to the full-sized image.
I hope that helps!
Regards,
Alec Smecher
Public Knowledge Project Team