[ubuntu-uk] Installing Paint applications 10.04

[Gordon Burgess-Parker]

Every Paint application I am trying to install from the Software Centre
(that's GNU Paint, mtPaint, XPaint, Mypaint, rgbPaint) comes up with the
error message "Requires installation of untrusted packages - The action
would require the installation of packages from unauthenticated sources."
I click OK and the installation stops!
WHY are these packages even in the Software Centre if that happens, and
how can I install at least ONE of them?