You should have no problem at all - you don't really even need an insulated bag. The lockers are all in the shade at all parks. It's really no different than your camera being on your person as you walk around. The bag might be a good idea just to disguise what you're putting in the locker, in the odd rare case some less-than-honorable person might get a temptation to attempt a locker break in. But otherwise, I'd have no problem with putting a camera in a locker at Disney, personally.