It is important that you pre-name your photos with a name that also identifies the photo. Bad Example: 1003.jpg / Good Example: cod_fishing.jpg. Then as alternate text for this picture called cod_fishing.jpg you might use- Example: Cod fishing out of Newburyport, MA
Just remember Text on your website is king. The search engines read text. A Picture is worth a thousand words to the viewer "BUT" if you don't get the viewer to the website it does you no good. You need to place alternate text on as many photos and graphics as you can. Use key phrases that you think someone would use to find your site.