Gimp is a lovely program but your method of using Word is actually far simpler (although I'd use DTP software, rather than a word processor, myself).
For Gimp, see here:
https://graphicdesign.stackexchange.com/questions/83446/gimp-how-to-combine-two-images-side-by-side
and/or here:
http://www.kalfcomputertechniek.nl/punbb/viewtopic.php?id=249
To stick with your existing method, it's best to have the Windows Snipping Tool readily to hand on your taskbar. (It saves having to find it every time you need it). So go to Start > Windows Accessories. Right-click on Snipping Tool and select More > Pin To Taskbar.
Then use Word (or a DTP program, such as Scribus) to position your two images alongside each other, as before. Click on the Snipping Tool icon that's now on your taskbar and then on New. Use your mouse to draw a rectangle around the composite image. Click on File > Save As. Choose a location for your file, give it a name and (importantly) check that the file type is set to Jpeg (unless you've a reason to choose a different format). Click Save. Job done.