Fotosearch is more of an industry portal as opposed to what stockers think of as an "agency". They don't take submissions etc, but rather form partnerships with established outlets. They are more tuned to the RM world rather than microstock. On a recent SEO project I did for a client, they rank in just about the top three across the board for searches relating to stock photography and stock video footage.

The fact that they have contacted you for linking means that they perceive your site as high value with regards to your positioning in the web world. They will likely not link back to you, but there is value for you in placing an outbound link to them, because FS is kind of a premium value site when it comes to web ranking. A glorified link farm of sorts that also provides value for photo clients. Only the Jupiter Images network is bigger. You may see a traffic bump up plus an increase in page ranks by linking.

They are a division of Publitek Inc. which is listed on MacRae's Blue Book, a premium general industry portal.

http://www.macraesbluebook.com/search/company.cfm?company=737362