[Plugin: Related Posts Thumbnails] How to just use the image? No resize.

I’ve just installed the plugin “Related Posts Thumbnails”.

My blogposts have one small image in each post, I would like to display that image in the related posts, not a resized thumb or anything, just the exact same image.
How can I do this?

Now all images shouw up as missing because the plugin adds “-150×150” or “-300×300” and those do not exist, I want to show the original images. How can I do this?