I assume that by 'take' you mean 'copy', rather than 'remove'. (Only your son, or Mum using his log-in details, can actually remove the pictures).
Any picture on the internet can normally be saved to your hard drive simply by right-clicking on it, and then selecting 'Save image as' (or whatever similar wording your choice of browser uses). Once an image is on your computer, you can then upload it to your own Facebook site.
If Facebook blocks 'right-clicking', you can still save any internet image by using a screen capture. The easiest way to do that is to use a utility such as ScreenPrint32, which lets you select the exact part of the screen which you want:
http://screenprint32.software.informer.com/
Chris