Image-licensing network GumGum is growing by leaps and bounds. Measured as an advertising network, it is now a Quantcast Top 100 site, reaching 13.7 million people in the U.S. and 23.5 million worldwide. More than 1,000 Web publishers are licensing images through GumGum, which allows them to pay based on how many people see the image or use them for free with embedded advertising.

Today, GumGum is adding blog network B5Media and gossip site DailyFill to its customer list. They join Glam Media, MTV, and Gawker. Sites that rely on celebrity pics particularly like GumGum’s model.

Sites contract directly with photo agencies, and GumGum keeps track of who is using what images and how many times they are viewed. That is what those Quantcast numbers are counting, thus GumGum acts like an ad network for images. Typical CPMs (cost per thousand views) are $0.15 – $1.00 per image for sites that opt to pay instead of accepting ads in their images. This kind of licensing model makes much more sense on the Web, where small sites want the same quality images as large sites but don’t have the resources to pay the same rates.