How to find advertisers for a niche that Google Adsense doesn't allow?

I own a fairly popular image hosting site (mostly memes). The content of the site isn't high value to advertisers on Google so my application to AdSense was rejected.

How can I find a way to monetize the site if most of the advertising networks won't accept the site? I feel AdSense is the largest of them all and being rejected seriously limits my ability to monetize on the service.

My biggest problem is that the cost of hosting images gets exponentially expensive as the traffic grows. I have been using Amazon CloudFront to serve the images through their CDN for speed, which further adds to my fixed expenses.

I could scale back on my costs by not using a CDN, but that still would leave me with no way to monetize the service. Any advice would be helpful.

There are plenty of ad networks other than Google's; my first step would be looking into what else might be a good fit. Probably networks that are less about contextual advertising and more about demographic targeting. BuySellAds is a popular network with a focus on display advertising, I'd try them. AdBlade is another very big one.

You can also look for ad networks or agencies that are specifically interested in memes / viral videos / etc. Some googling turned up Viral Ad Network as a potentially relevant platform.

Do you have pro accounts yet? Free image hosting with no limits isn't really sustainable; power-users will break you.