Our knowledge base explains in detail how to work with buttons in genARate. There you can read that an image can be added as the background of a button.
For the use of button images, the same rules apply as mentioned in the “Images” paragraph. If a button needs e.g. 30% of the width of the genARate stage (which is 300 pixels), it does not make sense to import a button image with a width of 1000 pixels and resize it in the editor.