Welcome, Ungrimmer.
It works for me. Is your plugin configuration set to overwrite Poster, and "Download images"?
If so, run the program with the -debug switch, and check the log (opened from the Help menu) to see what's happening.
BTW, posters from IMDb are not of good quality, and are intended only to serve as a temporary measure until a better one is downloaded using a poster plugin (e.g., Amazon, IMPAwards).