There was a time when all one had to worry about for optimizing their site was the text. There was also a time when websites were indexed manually. How times have changed. These days, text is still king of SEO, but integration of other technologies is becoming more important.
Photo and to a greater extent video now has a part to play. Yahoo! has featured universal search for a while now, but you might be surprised to know that they were beta testing it long before that. Universal search is the new standard and as Internet speeds increase, video will become the new web medium. YouTube is largely to thank for that, but do not be mistaken into thinking that YouTube is the be all and end all, you can host video yourself, you just need to make sure that you have the bandwidth to handle it..
Optimizing video is more important than optimizing images. Any video on your site should be optimized for search. People that look for video look for the same reasons that they look for text. They are looking for information and or for entertainment. Many people that are seeking out images are not doing so in order to seek out information. Video brings in the right sort of traffic. Chances are that if your site has video it does not have that much. If your video files are not properly tagged, you should go back and tag them. Optimizing your older photos might be more trouble than its worth if you have a lot of images. It is worth weighing up the potential traffic benefit against the workload. You might just want to optimize older key images and any new images that you post.