I played with this a bit and got mixed results.
Mostly only the audio files ended up in the Playing Now list, and the image files were shown as a slideshow while the audio played.
But once the image files also turned up in the Playing Now list.
It seemed to depend on whether I selected the folder in the left navigation tree of Windows Explorer, or in the right pane as a sub-folder. It also seemed to make a difference if I selected "Right-click > Play in Media Center" or "Right-click > Media Center > Play". But the former wasn't always available, while the latter was.
Anyway, I gave up on finding a consistent answer at that stage. It seems that the Windows Shell Integration could use a little cleaning up, and based on my early results, it should be possible to not have the slideshow automatically start. I did have that turned off in "Options > Image > Image Display > Automatically start slideshows", but that didn't have any effect on the slideshow in this situation.
BTW, I assumed that the image files being discussed were not imported into MC, and my example images were not, but still displayed in the slideshow.