1. Take screen shot using PrtScrn key or free proprietary (DirectX capable) screenshot software downloaded from the net. Good screenies tend to have just a couple of loons or vehicle or building as the main subject: long shots don't work.
2. Paste into Paint.
3. Then into your image editing software. (Or drop it in here directly.) Paint is not sufficient. Start - Programmes - MS Office Tools - MS Photo Editor is, just. Photoshop or Paint Shop Pro are best, but nowadays the trial versions do expire and are clever enough to detect a fresh download. They must leave a little file somewhere. Save your screenie as a .jpg.
4. From an existing mission such as the Tutorial Mission take the pic and open in your image editing programme. Hack it to bits using cut and/or crop commands until you are left with only the border. Save.
5. Open your screenie and crop to the desired size.
6. Add the border. It doesn't matter if the border and pic are different, you can copy and paste sections of border to make it fit. Spend a little time on this: it is fiddly but worth it.
7. You now have the perfect pic. Save. The imaging software will tell you the size in pixels, if you hunt hard enough. Use these size numbers in your line in the Overview.html document to define the size of the picture.
8. Give the pic sides in pixels which are a factor of 2. Exactly how you do this depends on which software you have. (Try Image - Resize.) Then save it.
2^1=2
2^2=4
2^3=8
and similarly 16, 32, 64, 128, 256, 512, 1024.
It doesn't matter which you use, although its easier if you use the ones closest to the actual size of the pic. Normal values for Overview pics are 64, 128 or 256. The important point is that when it is saved to these values the picture is distorted, but it doesn't matter because the command in Overview.html is what defines the size of the picture when you see it, thereby undoing the distortion.
9. Put the pic into your mission folder.
10. Use the Overview.html file from the Tutorial Mission as a template for your own mission.
11. Sit back, sigh with satisfaction and pat your stomach.