Smart Wishlist allows you to replace the default preloader image with a custom preloader image. Below are the steps:
1. Upload the preloader image (GIF) on any external server or to the Shopify theme. Note the URL of the image and test it by opening it in a new browser window.
2. Visit the following location in Shopify Admin:
Apps => Smart Wishlist => Configure Appearance
3. Replace the value of the field labeled Preloader Image with the URL of the image uploaded in Step 1.