We really cannot figure out why it won't accept your files. As far as we can see there is nothing wrong with them, but it does nor work, we also tried.
I cannot think of any other solution, so I have opened and re-saved the images in my graphic program and now it works with these files.
So all you need to do now is to save these images and upload them to the Wild Knuffel page:
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
I am sorry for this long-winded solution, but as I cannot figure out why it won't work with the original images, that's all I can offer.