When an image is uploaded into an MKDoc photo component two smaller versions are generated and stored for embedding as thumbnails in a page.
These thumbnails are in the same format as the original image, so a .jpg file creates a .jpg thumbnail etc..
This is fine if the original images are .jpg or .png, but it makes no sense if they are .bmp, .tif or .pdf files. The solution is to create all scaled images in jpeg format.