Setting a custom loading gif is a little tricky since the div containing the loader it added before any other Storemapper code runs.
You’ll need override it with your own CSS on your own site. This can either be in your normal CSS files or in a style tag on the page, above the Storemapper embed code. Storemapper’s Custom CSS feature (in Advanced Settings) will not work since the custom styles are not loaded until after the loader gif.
Note: you may need to use this trick to override the inline styles.