A: 

i guess its ur doctype... are you using strict? Just remove doctype and see if this fixes the problem.

Dennis
A: 

Put these two in separate button theme and it should solve your problem.

It's a wild guess but I checked it in MS-Word & Open Office they have it as separate theme as well.

Ravia
A: 

Version 3.2.2 of TinyMCE includes the changelog note:
Fixed bug where IE 8 would render colorpicker/filepicker icons incorrectly.

I certainly haven't seen this problem occur with the latest versions of TinyMCE so perhaps you just need to upgrade?

Regards,

Adrian Sutton
http://tinymce.ephox.com

ajsutton