Hi,
Especially the latest RT templates offer a solution for you.
1. In the Administration, go to Extensions > Template Manager.
2. There you click on your template.
3. If you are using one of the latest RT templates, scroll down and you will see "Show main logo".
4. Set this to "No".
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5. Save your template settings.
6. Go to Extensions > Module Manager.
7. Create a new module by clicking on the "New" icon on the right top.
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8. Choose Custom HTML and click next (or click on Custom HTML).
9. Now down the page you get your regular editor and put your image in there.
10. Most important, set your new module to the postion "logo".
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11. Save your new module
You're done! No background image, but a real image logo.