Okay the next thing to check is the image size that you are uploading to zencart is it a very large image that you are using for your products? If so resize them to smaller and see if that works

Also note .JPG is not accepted by facebook either it is has to be .jpg small letter extensions just something to check for.

I am using this module on several websites and I cannot see where the problem maybe as you see here all images are working and these were posted a few hours ago looking for any issues