I'd like to obtain 2D images of my peak clusters with the current slice shown in the corner of the image. I navigated to the correct coordinates using the Coordinates panel, but I can't find the option to include the MNI coordinates on the image. I have older images where the numbers are shown (see attachement), so I did accomplish this before but I don't remember how I did it...

The spatial coordinates and image intensity are shown in the titlebar. For the interactive views. For saving bitmaps for publication, I would suggest the mosaic view, which optimizes the bitmap for the image's resolution, rather than your screens resolution. The "L+" option will insert labels. Personally, when I created bitmaps I save mosaics without labels, and add them with an image editor or in the figure legend, as the automatic placement of the numbers might not ideal.

import gl
gl.resetdefaults()
gl.loadimage('spm152')
gl.mosaic("L+ C -69 S 34; A -9")