For me, the preview button on the forum doesn't open a pop up, rather it adds a new section to the page above where I'm typing. It can seem like it hasn't done anything, but if I scroll up slightly I see the preview section.
As a reference test, what happens if you open the local copy of the help manual
file:///usr/share/doc/litemanual/index.html
Click Start, then scroll down to an image and right click there?
Do you get the 'Save Image As ...' pop up as you'd expect ?
What browser are you using?
What version ?
Any plugins/extensions ?