I'm trying to upload a custom header to my blog, which uses the "Mystique" theme. The images I'm uploading are the exact size required, 940 x 200 pixels. I've tried uploaded both .png and .gif files, but they're not coming up as transparent. Instead, it changes the background of the entire blog to black. I really like the stock background that comes with the theme, and I was hoping I could use a custom header that has some transparency. Please advise.
The blog I need help with is smypp.wordpress.com.