What resolution for the photo?
Here is the answer, whether or not you decide to respond to this request. It is just as valid for other legit publications. But my personal advice is to NEVER give it away for just a photo credit. Your Film, Cameras, Time, Gas, etc., are worth something.
Ask them the size of reproduction. Example, 7" x 10" in the actual magazine. The scan MUST be at least twice the resolution (dpi) as the line screen used by the publication at size of reproduction. Sounds confusing right. Well here is the simple explanation.
As you hopefully are aware, all commercially printed photos have been screened with a dot pattern. Take a look at a magazine photo real close with a magnifying glass. You will see the dot pattern.
Most magazines use a 150 line screen (some cheaper mags use a 133), which means (for a 150) line screen, you must scan at 300 dpi with the output size set to their requirements. If they were using a 133 line screen, then the minimum scan would be at 266. For a newspaper on newsprint, most only use either an 85 line screen or at best, a 100 line screen. So a 200 dpi scan (at reproduction size) is ok for newspapers.
So when you talk to them about their requirements, just ask what size the reproduction will be? Then ask what line screen they use? Then go ahead and scan. Set your scanner to the dpi necessary, use the scale slider to set the photo up to the correct size and presto... hit scan. If for some reason your scanner only allows you to go to 200% (as a lot of scanners seem to do), well no problem. A 600 dpi scan at 100% is exactly equal in resolution to a 300 dpi scan at 200%. So a little math will always give you a solution.
This has several benefits. One is that you know the reproduction size. If they want to run your photo across two full pages with inset text, it would command a higher price than a simple two column pic. You can then price accordingly. Also, as you are not making a huge file that is not necessary, you can email the file as an attachment easier. I generally use JPG at around 8 compression (out of 10 in Photoshop) which although is a large file, it is still compressed some. This has been acceptable at up to double page size at 300 dpi.
For example, the last magazine photo I sent out was for a two page spread measuring 11" x 16.5". The uncompressed file size was 47 megs, but the jpg compressed file was 5.4 megs which was sent via email attachment. The actual pixel measurements was 4961 pixels by 3324 pixels at 300 dpi. The most important thing to remember is that you are dealing with their reproduction size and the line screen used. You are NOT dealing with computer screen resolution. Although screen is used to describe both, a computer screen and a reproduction line screen are two entirely different things.
Hope this helps you out. Any further questions, please ask.