Hide layers and show only either 51 or 52:
Now choose the group tool and drag a box over this dumb line in the bottom corner of your artwork
Hit delete to get rid of it. Then still using the Group tool, drag a box over the whole graphic. Because it's on the Docu layer and everything else is hidden, it should only select the artwork and nothing else. By dragging the Group tool box over it, it becomes the current group.
Now click the layers button again and choose Preset_top or Preset_bottom depending on where this is going. You should now be able to see everything on the top/bottom of your board.
Now click the Move tool:
Right click over your graphic and select "Move: Group" from the list:
You'll be able to move the graphic into position:
Almost done. Once you're sure you have it where you want it, hopefully you still have it as the selected group (if not repeat the steps in this comment), click the Change button, then click Layer...
and select either tPlace or bPlace
Now right click and choose "Change: Group" from the menu to move it to the silk screen layer:
You can see roughly how your board will look by clicking "Manufacturing" on the far right side of the screen:
Eagle is a bit of a pain when it comes to mirroring these, so if you want to place a graphic on the bottom side of the PCB, make a flipped version and import that instead.
Getting the sizing right involves some guesswork, I recommend changing the size of the image before you import it, alternatively you can change the scale factor when you're on this screen:
Your grid settings will probably change when you do this, just set them back to what you had before once you're done.
I'm sure there are better ways to do this, but this works well enough so it will probably help at least one person. Feel free to suggest better ways!