Use whatever options your Operating System provides. Not sure of MAC OS, but from Win 3 on there has been a 'screen print' option, via a key combination which will capture current screen image (and later on just the active window/dialog) to the copy/paste buffer. That will allow you to load it into a graphics package (MS paint works fine) via Edit> Paste and save it as a file. You could also crop/edit the image to remove any personal information you did want to be seen.
In Win 7 there is the SnippingTool which allows you to select the rectangular area of screen to capture.
Upload the saved image to one of the many free upload sites (dropbox, etc.) and give the link to the image in your message