Smart Wishlist allows you to replace the default preloader image with a custom preloader image. Below are the steps:
1. Select and download a preloader of your choice. You can use third party services like loading.io for this.
2. Upload the preloader image (GIF) to the Files section in Shopify Admin and copy the image URL displayed.
Settings => Files
3. Visit the following location in Shopify Admin:
Apps => Smart Wishlist => Configure Appearance
4. Replace the value of the field labeled Preloader Image with the URL of the image uploaded in Step 1.