You tagged your question with Sharepoint 2007 and 2010, the content editors are different though. This answer is for SP2010:

You can just add CSS to a custom CSS file. background-image for .ms-rte-layoutzone-inner-editable will do the image trick. If you want to add a "frame" as in border, you can add border attributes to .ms-rte-layoutzone-outer and make it e.g. red.

An example for a background image:

.ms-rte-layoutzone-inner-editable {
    border-image: url(/PublishingImages/Mylogo.gif);
}


But please do your users a favor and don't include anything blinking or distracting to the content editor's background - if you wanna go for some very light grey or something like that it's OK.

For MOSS2007 you need to check the specific styles you can override.

Sharepoint 2010. Add custom background and frame for content editor webpart
If you have the time and resources, what would be the most effective way to increase your salary as a full-time programmer, outside of just doing your job? 

By "salary" here, I mean salary (adjusted for location cost-of-living) coming from a single programming job. 
