Photos need to be 300 dpi to be published, and usually the larger the better. Tif is higher resolution than JPEG.
These type of projects usually take quite a bit of time. I am sure he is fishing for photos trying to get as many as he can to choose from, to pick the best from. The timeline for the project could be months and months or even a year or two away. The publishing industry works very slowly. I would suggest people be very patient and not be discouraged if you do not hear anything. There could be any number of factors including how far along the author is in the writing, Or it could be almost ready to go. The fact he is coming to you here is a good sign. It shows it is a very serious opportunity.
He is also the editor of TFH magazine. All three of the American magazines have a hard time hunting down photos for the feature articles. They usually keep a library pool of photos that have been submitted to them over the years to draw from, but even then they are always looking for something fresh and more relevant to the article. They only pay for the photos if they use them, and they may hold on to them for years until they decide to use them.
If you want to get your photos published, get chummy with some of the columnists or known writers. If a writer can provide their own source for photos, it makes the editors job much easier.
For example, in the May issue of TFH magazine, Rhonda Wilson had two photos from Roy Deki with her Planted tank column. I have used photos from several different people on the internet, each and every month of my column in FAMA and with my feature articles. You do not have to be a pro, you just have to have good photos of the right subject!