The image requirements for Google Shopping are:
Item must be photographed against a solid background, preferably white.
Submit the largest, highest resolution, full-size image you have for the product, up to 4MB file size.
Images must be larger than 32 x 32 pixels and cannot be larger than 64 megapixels.
We recommend images at least 800 pixels in height and width.
Do not scale up images or submit thumbnails. We recommend the product to take no less than 75%, but not more than 90%, of the full image
For apparel products, they require images of at least 250 x 250 pixels.
For more information about Google Shopping's image policy, click here.