filename (suffix) for images

Confused about the naming process for images when exporting to html. The thumbnail gets created with the id+.jpeg. The fullsize image gets id+_1.jpeg. But when doing this again later, the fullsize image (or at least some of them) gets a _2.jpeg suffix. Any idea how to get orange CD to always stick with _1.jpeg?

On my webpage there is a php script that looks for the _1.jpeg (the script can be re-written, but I rather like to know the idea behind the different endings on the filename)

Best Regards

Ulf