Oliver,
which HTML editor are you using (see default editor setting in your web.config or Host > HTML Editor Configuration). if you are still running old FCKEditor (default editor for DNN until version 5.3.0) you should switch to RadEditor or install free CKEditor (http://dnnckeditor.codeplex.com), which is the successor of FCKEditor 2. Please make sure to uninstall FCKEditor afterwards, because there are known security issues.

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Cheers from Germany,
Sebastian Leupold

   

Speed up your DNN Websites with TurboDNN

Are the buttons displaying at all just with the incorrect images?
What I had found in my instance of DNN7 was that the CSS created by Artisteer was wreaking havoc with the toolbar buttons for the RadEditor.
I forced the following CSS for ALL sites:

.reToolbar li:before
{
content:none!important;
}