Fritzing crops the silkscreen layers on export to match the edge of the board so there isn't really much for you to do other than place the image/part where you want it.
You can also make the image part of a custom pcb svg. The board file is comprised of a boardoutline group and two silkscreen groups (silkscreen and silkscreen0). You can put whatever you want on the silkscreen layers. Fritzing will also crop these images where they intersect copper pads and vias etc.
