In the .kobo/Kobo directory, I edited Kobo eReader.conf and added Screenshots=true under the [FeatureSettings] header. I normally use NickelMenu to toggle screenshots on/off since the power button is used to take the screenshot. This means you can't put your Kobo to sleep using the power button and would need to use a sleep cover to do this.
Sadly, since I have my Libra Colour on a v5 firmware, NickelMenu will not run.
The options I use with [FeatureSettings] are all below though Screenshots is the only one you need. Some are just leftovers from testing various items.
Code:
[FeatureSettings]
BookSpecificStats=true
ExcludeSyncFolders=(\\.(?!kobo|adobe).+|([^.][^/]*/)+\\..+)|(screensaver_old)
FullScreenReading=false
IgnoreCaptivePortal=false
InvertScreen=false
Screenshots=false