Here's my process for creating screenshots suitable for upload to OpenMMO...
I personally use Fraps to grab the screen, but the ingame screen capture tool would work equally as well.
Once you have the image that you'd like to edit, bring it up in your editor. I use Paint.NET. It's fairly powerful and free. http://www.getpaint.net/index2.html
In Paint.NET, make sure you have your Colors(F8) and Tools(F5) bar up.
Change the main color to white.
Press the "O" key four times, to bring up the ellipse tool. Clicking it may be easier. :)
Make a circle around the location that you're trying to identify.
If it looks good, Press the "S" key to bring up the "Rectangle Select" tool.
Highlight a decent amount of area around the Point of Interest that you circled. You want to make sure that people will understand right away where this is located, but without selecting the entire map.
Once the area is selected, Press Ctrl-C to copy it. Then Press Ctrl-Alt-V to paste as a new image.
Save that image with Ctrl-S.
The default image format for Paint.NET is PNG. PNG is a nice format, but it doesn't seem to compress as well as JPG for what we're trying to do. So change the format to JPG.
Make sure the name has no spaces in it. There's a type of formatting called "Camel Case". It means that you Capitalize the first letter in each word and leave out the spaces. i.e. YouPokedMeInTheEye.jpg
After you click save, it will ask you for the compression amount. This will vary wildly. For a small map, I adjust it between 51-66%. This will often yield an image of 15-30k in size. The smaller the size, the better. But make sure that any included text is readable.
Now just upload it and link to it.
Quick Gimp Method:
Swap the primary color from black to white.
Press O to get the circle select tool and select the area you want to highlight.
Press Ctrl-O to bring up the stroke tool. Change the pixel size to 3. Click ok.
Press R and select a rectangular crop area.
Press Ctrl+SHift+X to remove the unselected area.
Ctrl-S to save. I've been saving them at 55%. We don't need a real hi-res image.