If you have a picture of a stain that you want, what you can also do is apply it to your image in it's own layer with a bg of white... using ctrl+alt+g to clip it to the layer below, then use the blending mode HARD LIGHT or PIN light. Play with the opacity, and you will get a stain. Add a few effects like gradients, glows and play with their blending styles...
Here is an example:
Sandeman