Repeater integers

Is there any sort of integer I can use for a repeater in an HTML page? I've got a template with a repeater, but I need to change the css ID tag.

I've got a background-image for the css that changes each time, I can't put it in a style tag as it uses an @media command, so I'm trying to do something like

div#part1, div#part2, etc

But I can't work out where to get the 1 or 2 from within a repeater.