Export map image in QGIS using a precisely defined area

I have to continuously export map images (of different layers) from QGIS to use them in Photoshop for further processing. Given that Photoshop can’t deal with world files, I need to export the images in precisely the same size, scale and mapped area.

My workflow is to store a QGIS project for every map project. Reopening the QGIS projects and exporting images gives perfectly fitting images that can be stacked in Photoshop. This setup is rather fragile, though. Moving the map in QGIS just a little bit and accidentally saving the project leads to images that don’t match any more. The same happens when the QGIS window size is changed.

I tried to copy and paste the values (like “Extent North, West, East, South”, “Scale” and so on) from the “Save map as Image” export dialog, just to find that the exported maps aren’t always the same size. Perhaps this has to do with some rounding errors.

So, how can I export images of a given map that have precisely the same size, resolution and covered area so that they can be stacked in post processing?