You can let the user know about the expected dimensions and then crop/resize the image for them if it is not the correct dimension (without the user having to do anything).
You can also implement a UI for the image upload that lets the user crop the image in the browser. I don’t really know too much about e-commerce CMS systems but I wouldn’t be surprised if at least some of them had such an image upload implementation with a crop feature.
Not sure if this project is using a framework or plain HTML/CSS/JS but I’m sure there are pre-made libraries/components with a UI for doing image manipulation you can use. Even with help from a library/component, it is obviously more work, but it’s also a nice feature to have.