Q: Cracked iMac screen - Retrieve photos.

Don' t you have a backup of the old machine? If you do then it's simple thing to migrate over.

You can take the hard drive out of the iMac and install it in a suitable enclosure. Then connect that box to the new Mac. At that point the old hard drive will appear as an external drive on the new mac, and you can move the photos over.

And, as an additional benefit, you will have an external drive to use for regular back ups.

You have to verify the type of drive that is in the old iMac. Unless this is an ancient iMac, it is a SATA drive. In which case this is an example of an enclosure you can use:

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=39500 07&CatId=2780

You have a 3 1/2" SATA drive, acording to these specs:

And an apology. I gave you a link to a USB powered hard drive enclosure, i.e. one that gets its power from the Mac's usb port. I avoid these like the plague. You are better off with an enclosure that is plugged in to the wall for its power. Else your Mac could simply, without warning, disconnect the drive because it is drawing too much power. A better option: http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=13990 58&csid=_61

How to use and troubleshoot FireWire target disk mode