Defining a background image of the editor toolbar?

i saw, that sitecake has a background image defined for the small editor toolbar:
“data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAYAAAAoCAYAAADdaosOAAAAGElEQVR42mMwNTX9jw0zjEqMSoxKDBcJACKzhDBhoILGAAAAAElFTkSuQmCC”

how and where can i define my own image?

cheers!