Google uses wrong image to display product in Google Image Search

I have a problem with Google associating the wrong image with our products when they are display in Image Search.

For some reason, the main image of our products, which has more desirable data (alt tags, higher resolution, placed first as product image, not tampered with), is discarded.Instead, Google chooses the last loaded image for a product, which has no alt data, worse resolution, watermark dynamically added (it’s not even .jpg), or just blank white image with the watermark.

I have Schema.org microdata set up dynamically for each product. I’ve removed the "additional_image" tag, using only "image", still no success.