Plotting on top of an image without actually showing either

Hi guys Basically, I want to show an image, plot something on top that image, and then save this over-plotted image. However, I want to do all this without ever actually showing the image to the user.

I realise I could directly modify the pixels in the image file but this is laborious and slow, and I don't have access to the plotting tools. Is there a better way of doing this?