>  Are urls to uploaded files/images persistent or do they change ?

Yes, they are uploaded through the image to collect some meta-data,
but then super efficiently served through a static Apache URL. So you
can upload huge pictures (or files in general) easily without putting
any load to the image. I already wanted to build some image processing
for a long time (at least the resizing part), but didn't find the time
to actually do so. Maybe today while traveling .... ;-)

Lukas

-- 
Lukas Renggli
http://www.lukas-renggli.ch