Great question! Every theme will handle or display its images differently. Meaning the recommendations and best practices for the sizes and aspect ratios may differ. My suggestion would be to find a theme that you like, that has the functional features that you need. From there, most times the developer of the theme will have documentation regarding their theme, which will include best image practices for using images within their theme. I recommend checking out our theme store here that has themes specialized for art & photography-based stores. In there, you will also find a few free theme options.
I hope that helps! If there is anything else I can help you with, please let me know.