The photoshop file should have either slices or guides setup. If not, it isn't a very good theme. The other thing I've done in the past is to open the jpg image that I am going to edit (the one that is used on the website) and open up the psd. Make the edits that I want to that section, then create a new layer on top of all. I press CTRL + ALT+ SHIFT + E, which makes a copy of all visible layers on that new layer. Then I drag that layer to the jpg I had open and lower the opacity to about 60-85% and move it around until it is positioned just like the old image. That's what I do, anyway.