Your photos look best on Google if they meet the following standards:
Format: JPG or PNG.
Size: Between 10 KB and 5 MB.
Recommended resolution: 720 px tall, 720 px wide.
Minimum resolution: 250 px tall, 250 px wide.
Quality: The photo should be in focus and well lit, and have no significant alterations or excessive use of filters. In other words, the image should represent reality.
See the information directly from Google: https://support.google.com/business/answer/6103862?ctx%3Danstogmb