Alexander,
Take a look at Christina Obry's tutorials on SCN, particularly the one entitled Branding and Theming of BI launch pad. She goes into much more detail than just changing the logo. Working with a JAR file and CSS may seem intimidating at first, but it's really not too difficult.
The method she describes is also fully supported by SAP. In addition, your JAR file will be preserved whenever you patch your system.
Best of luck,
Dallas