Quick Mixer can use ANY .BMP (Windows Bitmap) file as a skin.

The file must be named "skin.bmp" and exist in the QMixer folder.

Seamless Tiles make the best skins.  

You can find THOUSANDS of seamless tiles on the internet!

Just go to http://www.yahoo.com and use the search-term,
"seamless tiles" to find them.

It doesn't matter what file-type they are!  You can save
them and convert them to the .BMP type in one easy step.

If you are using Internet Explorer as your browser, just
right-click the picture you wish to use as a skin.

Click "Save Picture As.." on the menu which pops up.

Change to the C:\Program Files\QMixer folder.

Change "File name:" field to "skin".

Change "Save as type:" to "Bitmap (*.bmp)"

Click the "Save" button.

Now, just toggle Quick Mixer's "Show Skin" option off and
back on.  

That's all there is to it...