I can think of at least 2 ways, one being for free, and the other will cost some money:
1. Free method - you can EMail the pictures that are on the iMac to yourself. After you send the pictures, start up your Mac Book Air, and launch your EMail program there. You should see the pictures in your In Box.
2. Depending on the exact Mac models you have, you would need to purchase the appropriate cable, connect the 2 Macs, start up the Mac Book Air, then startup the iMac in Target Disk Mode by holding down the T key while it starts up. The iMac's internal drive will then appear on the Mac Book Air's desktop, and you can then retrieve the pictures from the iMac's drive.