Update:OK about ThickBox, I found the version that supports button but I get a message saying "Operation could not be completed" when I try to use the controls and add them in design mode. Not sure what else to do :( Any idea why I get this message?
I already found the Ajax Control Toolkit, but I can't install it for some reason (with Visual Studio 2008). I also found ThickBox but I couldn't figure out how to trigger a lightbox effect pop up window by clicking on a C# Imagebutton control. It needs to trigger an ASP .NET page.
Any other suggestion?