1. Sign in to the Advanced Editor for your store. Then create or edit a link on your front page.

2. On the Edit page for that link, select the "New Property" button at the bottom of the screen. (The New Property button only appears in the Advanced Editor.)

3. Create a new property named "Icon" and of type "Image." (It's not possible to delete a new property or edit the type of a property once it's created, so you'll need to be careful to use these exact names and types.)

4. Upload your custom button image for that link to this Icon property.

5. Update that Edit page, then click on the editor's "Variables" button.

6. Set your Button Style Variable to "Icon."

7. Update your Variables page.

This will cause your navigational buttons to display as icons. The images used in the various Image fields on your Variables page and in the Icon fields of your Sections, Items, and Links will be displayed at their full original size. If you do not have an image for a particular Item, Section, Link, or pre-defined button, then the button will appear as a text button.

You will need to publish your store site in the editor for any changes to take effect.