I'm 3 years late, but thought I'd mention a few other techniques that weren't mentioned in the FAQ (in case someone is doing a 'search' for info related to this topic) ...
If you want the logo to occupy the entire background of the graph, the easiest way is to use goptions iback=
If you want the logo to occupy the area within the axes of a gplot, you iframe=
And if you want to programmatically control the exact placement and size of the image, you can annotate the image.