[Borgs8472]

If you don't mind the thumbnail actually being the whole pic then resized, then you can use this bbcode:

<a href="{param}" target="_blank"><img src="{param}" width=150 height=150 border="0" alt="thumbnail picture, click for full size"></a>

That will force the pic into a 150x150 square. It will take as long to load as the full image however.

There is a way of specifying just the width or height and keeping the aspect ratio, I can't remember it off-hand.

You could replace one of the the 150's with {option} if you want more user control to maintain proportions for instance, however if the user enters a very high number, it can easily screw up your forums so is a kinda security risk.

Enable the multiple options in bbcodes hack that's about if you want options for both height and width.