Cool 
Did someone forget to activate this? Sounds like just the thing someone might overlook somehow.
More research in the code, reveals that my original assertion that it uses fastmem when using the CPU based blit lib was 100% true.
I can open a 1920x1080p image on a WinUAE setup with only AGA (ie no uaegfx based RTG) which is as close as I can get to my retired a1200.
Such an image uses 1.97MiB of mem, which allowing for workbench at 256 and ppaint screen itself clearly doesn't fit inside a 2Mb limit.
In fact I have 298,984 bytes left over.
However one issue I did see is that intially it throws up a requester saying insuffiecient Chip mem for the requested screen, this is because it tried to allocate a max overscan super hires laced 256 colour screen. It then fell back to a simply None Overscan Hires-Laced
Max overscan off 1472 x 566 = 833152 bytes so maybe a bit tight with the WB and any background images open at the same time.
Some fine tuning to predetermine the display mode in some future version might help with this. (if there isn't already an option I missed)
There may still be an issue with even larger images.
BTW which version were you using?
