Select the page/element you want the new button to be on. Hit the INSERT Key on your keyboard. Add the new graphic to the generic grey button just created in the usual way. As long as the imported graphic does not exceed 320 X 240 in either direction the button should re-size itself. You may have to adjust the buttons fore-to-aft position on the page, by right clicking on the button and selecting 'move to front' or 'move to back' to achieve your desired effect.

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