Recommended Tests

I would say try it for a little bit... While on one hand since you run an image site.. the images will load faster..  But on the other hand keepalive stores the cache into memory and then doesnt release it for a certain period of time..  For you this would be a huge benefit because you wouldn't have to run as many daemons for each individual request.. but if incorrectly set up.. your memory is going to fill and your server will be brought to its knees.

But that being said.. You need to watch how long you are keeping the session open..  Too long and you are going to kill the server and loose clients..  to short and you are going to see little/no benefit.   The final aspect is the number of allowed connections..