This article only applies to the old designer. Not sure which designer you have? Check here.
Various elements on the design templates work best with images of a certain size or ratio. There is an easy way to check to see what image size you should use.
Locate the image you want to change
Every image has a little icon in the top left corner. Click on the icon to see the current image size as well as suggested image size.
Note: Make sure you're image is less than 5mb before uploading.
Related Links: Add Product Images • Add Product Variant Images • Edit An Image