Photography: repeating designs

I moved the camera when I took a picture of some fairy lights outside last Christmas. This was a small section of that photograph.

Then I reflected and repeated it to produce this. I used to do this a lot with images I created from scratch on DPaint, and always wondered if anyone has produced software to do this many times with one keystroke. I guess everybody tries this when they get a paint program so I'm not doing anything spectacular, but I'd be interested to know if there are any tessellation programs you'd recommend to speed things up.