Ok, I just tested everything and I think this works as expected. 
Here’s a new forum page I created with the picture input and workflows to save the image to the Current User. You can test everything as well in the forum app and it will save to the Current User (even if you’re logged out of the forum app).
Preview:
Editor:
With data:
In summary, each repeating group has conditional formatting which changes the repeating group’s data source based on the Group number of the User’s most recently uploaded image. This way, the most recently uploaded image is always in the top left cell in the 1st column (similar to uploading something to Instagram). Each repeating group involves a lot of conditional formatting statements, but I think the loading time is still fine and each repeating group column updates very quickly after each picture upload.