Ah! I had forgotten to validate my code... I checked it in Firefox and Safari on Mac, and FireFox and IE 6/7 on Windows and figured that covered 97% of browsers. So it functions, and therefore validity is not too terribly important, but it is something that I would like.
Also, I have the back-end set up to allow the user to get a larger version of the images, or to display a bit of descriptive text with each individual image, but haven't decided the best implementation, and I wanted to get the website live ASAP.