I had thought that all the images we saw of the initial terrorist attack into Israel came from Hamas...but it seems that several mainstream media outlets were the ones taking the photos:

"Mainstream media outlets" weren't the ones taking the photos.

If you read the article, these images were taken by "freelance journalists" - all be people who live in the Gaza Strip and submit work to media outlets as freelancers, not agents or employees of the outlets.

Gaza's a very narrow place. Half the area is within a ten-minute run to the Israeli border. It is not the least bit surprising that freelance photographers in Gaza would be able to capture images of the attacks, and then send them on their own (not previously commissioned) to the wire services.

The same thing would happen if the event wasn't an attack, but an earthquake or tsunami some other natural disaster. If the area is newsworthy, there will be a non-trivial number of freelancers among the population that have connections to the wire services to get photos out. That doesn't mean they were "collaborated" with the earthquake or tsunami.