The basic steps are these:
1. Take a screen shot
2. Paste it into a graphics program
3. Cut out the mini map from the lower left
4. Save to .PNG, .GIF, or .JPG
There is no hot key combination to take screen shots in SH2. You must use the PRINT SCREEN key on your keyboard. This loads it into the Windows clipboard. If you have SH2 running in a window use ALT-PRINT SCREEN to capture just the game window.
The Paint program that comes with Windows will do if you don't have a separate graphics program. If you're running full screen you can temporarily ALT-TAB out of the game to access your graphics program.
The size of your mini map will depend on which screen resolution your game is running at when you take the screen shot. The higher the resolution, the larger the resulting mini map. A screen shot of the game running at 1280x1024 makes a good sized one that fits well with the download pages and shows good detail. If you can't run your game that large, 1152x864 is a not quite as big, but still pretty good. 1024x768 is a bit small but will do. Any smaller is really not good.
It's best to show the entire map. If size is a problem it is possible to take multiple screen shots of different parts of the mini map and stitch them together into one map, but it's a lot of work. That's the method I used to create estate maps for the walk-throughs.
Be sure to adjust the zoom level of the mini map before taking your screen shot so that the map completely fills the box, but none of it is obscured. There are 11 zoom levels available with your mouse's scroll wheel. The fourth level up from the smallest will just fill the box the map is in. There is also a zoom button in the extreme lower left, but it only provides three zoom levels and, unfortunately, the one that just fills the box with no cropping is not one of them.
DoY's infamous TREASURE ISLAND - Challenge #13 - Sparrow's Extreme Challenges for SH1 and Crusader