How to make images change on my product page when a customer clicks on a different product option e.g. colour?

To make sure that your product image matches up with the product option that your customer is browsing, you will need to start creating your product with all the different variants then press save in the top right hand corner. 

Then scroll down towards the bottom of your new product page in your Shopify admin. You will then be able to see a list of the different options (aka. variants) for your product. There will then be a square to the left of your product option title. Click on the blank square which will enable you to upload the correct product image to match the variant. Note that you will need to have already uploaded the correct images to your product page before being able to select the correct image for each product variant.