Okay, ignoring JPEG because I probably accidentallly imported image as background instead of mask, here's steps to do it on a BMP:
1. Take a screenshot, open it in Photoshop CS6:

2. Spend 5 minutes trying to cut it pixel perfect to the desired resolution:

3. Save as a BMP... but not be given any option to do 8bit. Hmm. Well, see how it goes:

4. Not great - AGS rejects it for having too many colours.
Okay, import it into Paint.exe:

5. Be warned that this will change colour quality (why?). Import into AGS, be given this error:

6. Be left with this. Bravo.

Okay - let's take photoshop out of it. Let's import the screenshot as a BMP, and save it in Paint.exe as an 8bit BMP:
1. Take a new screenshot, saved as BMP, open in Paint.exe

2. Grab the area of the actual background; I did it free hand, and did it pretty nice. This a 1024 background on a 1920 screen.
But there are some bad grey bits about 2 pixels out of the edge of the image. Since I didn't believe I could get the proper image, I zoomed in a bit to make it easier; and then when the select area didn't scroll for me, I was stuck:

3. I saved it as a BMP 8bit, preparing to open it in Photoshop, zoom and cut to exact size, then select the whole area and paste it back into Paint.exe.
Except then it said that "the colour quality will be reduced", and I was back where I started.
So then I drew on the image, expressing my anger, and threw the entire thing out.

This took 45 minutes.
So please explain to me: WHY not write the 25 lines of code to export the walkable areas/regions/hotspots/etc. as an 8bit BMP file? WHYYYYYY?
Also, this is ignoring what would happen if I had 1920x1080 backgrounds; which was what I was trying to do in the first place! There wouldn't be enough screen realestate to do a proper screenshot.
We could just get rid of all this mumbo-jumbo bs, and instead of trying an exact sequence of events, we could just ADD AN EXPORT BUTTON.