First off apologies this is probably in the wrong area, I couldn't see a section relevant to either canvas or drag/drop behaviour, feel free to move accordingly.
I'm experiencing slightly strange behaviour when dropping images onto a canvas: The first drop has to be very accurate, ie cursor near to the centre of the canvas, subsequent drops overwriting the current image however function as expected with the cursor somewhere within the canvas.
Please see linked image for an idea of the canvas area that images are being dropped to.
The entire area including border, and area where image name appears allows for drops. However like I said the first drop you have to position the cursor more closely to the centre of the area than subsequent drops.
I can't see anything within the code to suggest the first drop is being treated any differently to others.