Giving information to a image

I got a (maybe a bit silly) question. I'm making a photo gallery using javascript. I need to give every image some information (a id), so i can ask javascript what image is currently open.

Example:

I got a array of some images. Those images have a id 1, 2 and 3. When I'm using the gallery to slide through the images, I want to know what image Im looking at.

It can be done by giving the images alt the information I need. So I can go like;

var id = document.getElementById("image").alt;

But I though this wasn't the way to do it. Is there a simple solution for this problem?