When trying to solve problems that arise using Google Sites, sometimes there really is no substitute for being able to actually see what is on someone's screen. Providing screenshots can be easily accomplished. Listed below are a number of ways. This list is by no means exhaustive but should provide enough help for most users.
My preferred way of sharing static screenshots is to use a browser extension. 99% of the time, I use the Chrome browser and two tools that I recommend for this are:
Both of these allow you to capture a screenshot and share it online.
If none of these take your fancy, have a look at http://www.hongkiat.com/blog/screen-capture-tools-40-free-tools-and-techniques/ and see if there is something there that suits you.
There are many ways of capturing a screenshot from of which the possibly the simplest (for PC users) is pressing <PrtSc> which puts an image of your screen in your clipboard which you can then paste into another application.
If you have managed to do a screen capture and save it locally to you computer, then you need to upload it to somewhere where the people helping you can see it. Since people looking at this 'how to' will be people who are using Google Sites, then there are two obvious solutions to this:
Another option is to use Jing (also does screencasts) which gives you the option of sharing your screenshot on various websites - but you do need to download and install it on your computer.
For those situations where a static screenshot is not enough, consider a screencast. My current tool of choice is screencast-o-matic or Screencastify (Used to be Screenr. Another option I have used in the past is Jing which also does screenshots - see above). No download required.
I upload to Drive to share screencasts online. e.g.