Buttons would need to be in:
PNG format for still
APNG format for animated buttons
300x300px
Step 1 - Select your specific event and go to the section titled "App Experience" and select "Edit".
Step 2 - Select the option "Customize Experience Buttons" which will then open up the "Experience Button Customizer".
Step 3 - Hover over the button that you would like to customize, then select the cloud upload icon to upload your PNG or APNG. There is also a delete button to revert back to the original Snappic button and the back arrow will revert back to the previous artwork that was uploaded. Once you are done uploading your artwork for the buttons then you can click "Done".
Pro Tips for experience buttons:
In case you have a small animation that you would like to upload as a button I can recommend using EZGIF.com and selecting APNG*, From there you will be able to upload any animation and create an APNG animation that can be uploaded as a button.
Also, note that all of the buttons are displayed on the Attract screen only for the settings part and that buttons can be removed from the app by selecting the experience that is not needed.
*APNG - This file is an animated GIF that can be transparent