Facebook require images to be at least 200 x 200 pixels in size, so if your product image is too small it won’t appear when sharing on Facebook. Also, Facebook will ignore any images 8mb and over, so if you have huge images, please reduce them in size before adding to the website via a free tool like picresize. Also having large images on your product can increase website page load time and frustrate your potential customers.
However if your image is the right size, it might just be Facebook being buggy! To force it to look for your image you can use their Facebook Debugger Tool. Simply copy and paste your product link where it says ‘Enter URL’, click ‘Debug’ button, then click ‘Scrape Again’ button. You may need to press ‘Scrape Again’ a few times for it to find your product image.