MacBook Airs:
extremely portable
slower processors than the iMac
SSD is faster than the iMac's traditional hard drive, but less storage capacity
memory can't be upgraded after purchase
no Thunderbolt ports
iMacs:
not very portable
faster processors than the Air
traditional hard drive is slower than the Air's SSD, but has more storage capability
memory can be upgraded after purchase
has Thunderbolt ports
Either machine is capable of photos and videos.
The iMac is going to have a faster processor, so it's going to be faster at CPU intensive tasks, like converting/export video. For me, I don't do these tasks often.
The Airs have the solid-state drives, which are insanely quick. This can make the Air feel like a faster-overall system when doing normal daily tasks, like surfing the web, email, iTunes, etc.
There's really no clear-cut answer. You're going to have to figure out if you need something portable or not, and if you want a Mac with a faster processor or a faster hard drive.
For me, the slower processor of the Air was outweighed by the very fast SSD. I'll gladly wait longer for the Air to complete CPU intensive tasks that I do a few times a month in exchange for doing the tasks I do daily much quicker than any other Mac I've ever used.