You can manually adjust the indices and overlap with the main image. Note that the main image pixel values will be lost in the overlap portion. Considering main image is larget than marker image.
Next: Find the coordinates where you want to place the marker in the main image. You can find data of approximately middle rows and columns from R and C. Once you get the coordinates, the next task is to put im_data in the main_image as I did in the marker (last line of code)
Note: A fairly simple problem, first use pen and paper to decide the coordinates.
Hope you can do the next step. :)