I have an image site with around 300 pictures. Giving me the benefit of the doubt to say they are good pictures, how many of the 5000 could I expect to see return?
I am afraid that if your site provides 300 pictures only without adding new fresh ones every day, even they are good, interesting pictures, returning rate will be very low.
repeat visits will depend to a large extent on your updating of the site constantly. Sites whether they are content based or image based will get repeat traffic if the are frequently being updated. No one wants to visit an outdated content site. So keep updating your site and you'll see many repeat visitors to your site.
Return rate really depends on where do you get these visitors from. If they watch your pictures from Google Images search - return rate will be very low --> 0%, since there people even do not check the website name normally. If you get visitors from standard Google search, and frequently update your website with new pictures - you might get high return rate --> 10-30%.
are thye buying the pictures or are they used to get traffic to increase your adsense campaign? try promoting samples on a Facebook page then give them more on the real website and get them to sign up for a daily or weekly e-mail list to keep them updated on new pics