The photos syned to the iPod are very low res versions of the original photos, optimized to fit on the iPod's small screen while taking up as little space in storage as possible. They are stored in a single special database file. You cannot use iTunes to transfer them to the Mac, and if you could, they would look bad blown up to a larger size on the iMac's screen.
There is a setting on the iPod's Photos screen in iTunes (a checkbox) that says Include full-resolution photos. IF that setting was checked when you synced the photos to the iPod, you can Enable disk use and find the originial full res photo files in the Photos folder (on the iPod).
Otherwise, you should transfer the original photo files from your previous computer, if possible.