Thanks for checking out the beta version of WP Tiles! :)
I had a look, and I can't tell why it's coming up wrongly. This is the post that's not showing up correctly: http://www.emmaus-apeldoorn.nl/beeldspraak/beeldspraak-5/ .
I see that you have the images in the post content - this is only the last place WP Tiles looks for the image to display. First it checks for a featured image, then for the first attached image, and if all else fails for the first image in the content.
So the first step would be to check if this image is perhaps set as the featured image, or is attached to the post I mentioned above. To check if it's attached, go to your Media Library and look for the image. There you will find what post it is attached to.
Let me know if this yields any clues!