Designing Mac Desktops Problems!?

So just started working with a new G5 imac and im having trouble with
desktop images and resolution. Ive been making my images at the set
screen resolution of 1440 x 900 but when i set them to the desktop they blur
and smear out … so whats the deal ?
these images are being created in illustrator and im going for that clean
vector look and all looks fine with the same images just modified image size
to 1280 x 1024 for my pc monitors.
so is there some magic trick with the mac because im lost?
Thanks for any help.