There are many Joomla extensions available (for example slimbox,
lightbox, shadowbox) that add a "pop up" feature to images shown on
a page. The effect is usually that when you click on a thumbnail,
you see a slick Ajax effect popup box appear with a larger
image.
All of these extensions use Javascript to achieve their effects.
There are however, several Javascript libraries, for example,
JQuery or Mootools. Additionally, Joomla itself comes with Mootools
pre-loaded and occasionally a template might use Javascript.
Often these libraries can conflict with each other causing
errors, or an extension not to work.
Whenever you are using such extensions, you'll need to make sure
any libraries you are using are compatible. The best thing to do is
usually to pick on and make sure all your extensions and templates
are compatible with it.
Its relatively easy to test, look for code in the head of the
template index.php that is loading the library, and then comment it
out.