User captures full images in the field and syncs them to the server - i am trying to remember the workflow to make sure that images once captured and synced up only thumbnails are ever synced back out to other users. 

My first instinct would be to have pictures and thumbnails in separate tables, and configure the full image to be a one-way sync from spoke to hub, while the thumbnail table would be configured as a bi-directional sync, with some sort of scripting on the server side that would automatically generate thumbnails and thumbnail records from the full images. If this doesn't sound like it would work with your situation, let me know some more details about your database, and we should be able to figure something out!

I agree with Evan. Remember that if you want to, you can also use the MirrorSync customization script and trigger after inserts and updates to generate the thumbnail in the other table.