I'm just starting to ad image ads to my campaigns. Should I add them to the existing adgroups or should I keep text ads and image ads separated?
Well, I'm going to assume you've got your search and your content campaigns completely separate already... It's always a good idea to separate things out as much as possible - it just gives you more control to react to what is and isn't working, and make changes.
You will definitely want to keep your image ads in a different campaign. They will get a much higher ctr than your text ads on the content network. You don't want the lower ctr of those text ads affecting the history of your placements.