PNG files ALWAYS look better than jpegs.
The compression is completely different.
A jpeg will exhibit artifacts where a PNG
likely never will.
If your pictures are 7k wide that is way too big
even for cropping and panning. You should do
a batch/action in photoshop and get them down
to 2k or 3k in size. That will likely not
cause any issues and allow you to get the job
done without crashing.
Even a robust compositing app like After Effects would
have problems choking down many 7k images.
It’s just not practical even with 3Ghz+ cpu’s and 4 Gigs
of ram in your system.
(I do a lot of this kind of thing btw and this has been
my experience)