An administrator can go to Special:ThemeDesigner (it'll be on your tools in the toolbar too) and upload a logo in the Wordmark tab. The logo must be 150x65px, and as I recall must be a .PNG image.
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You would like to use a portable extension such as: .png
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portable network graphics or png for short supper
transparency
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PNG is an image file format, such as JPEG or BMP. One of the
differences between PNG and JPEG, is that PNG supports
transparency.