I will show you another method to select a part of that window and save it to a JPG file.
Steps 1 - 4 can be done with the acquire function mentioned above. The reason I'm showing you this method is so you can get familiar with the size options. The ability to manipulate the screen size in pixels as well as inches is unique.
1.) Determine you display resolution by right clicking on the screen and choose PROPERTIES, then SETTINGS. From the screen capture below you see 1400 x 1050 resolution.
2.) CTRL PrintScreen - captures your entire screen to the clip board. I captured my writing this blog post with the gimp toolbar visible. I will cut out and save just the GIMP toolbar next.
3.) On the GIMP toolbar select new image and set the box size to equal your screen resolution.
4.) Paste the image into the new box. (this can also be done using Print Screen Button and the "acquire -> then paste as new")
5.) Now you have a full screen image. To select an area you need to select Layer from the GIMP menu and Anchor Layer.
6.) Now you can use the selection tool to select an area and cut or copy it.
7.) Go to your GIMP tool bar and select new again and the window will pop up in the exact size of the area you just cut or copied.
8.) Paste it into the new window.
9.) Now just save it to the file format of your choice and you are done.
In my case I chose JPG and Export and I was done.
The process seems long but takes about 10 seconds. If you do a lot of this I would suggest you use one of the screen capture utilities I recommend in an earlier post.