You will need transparent png images to create your customizer.
All your images must have the same size. The recommended image size is 1200 x 1200 px.
You need to basically cut out every customizable area of your product, see example images below.
Applying colors dynamically on your product
Instead of creating multiple images for every color you are offering, you can simply desaturate your image and use Kickflip to apply colors dynamically on them.
Loading different images to display a variation of your product
If you want to show a texture variation or a shape variation, you will need different images. For example:
Textured images examples:
For each answer, you can upload a thumbnail image. Thumbnail images are displayed in the Question Panel.
Your thumbnails images should be squared and their size should be minimally 50px x 50px but no larger than 300px x 400px.
Thumbnail image example: