Create the button images separately in a photo editing software like Photoshop.
The default PBU buttons (except for Home and Share buttons) are 315x102 pixels. New buttons will be automatically resized to these dimensions (without skewing them), unless they are set to a scale other than 100%. The buttons have a normal state and a pressed state. If you don't make a pressed button graphic your button will not change when pressed.
For great quality buttons we usually make custom buttons 630x200 (Accept, Edit, Photo, Morph, etc.), Share buttons 709x118, and the Home button 175x175.
All these sizes assume your monitor resolution is 1920 x 1080.
1.Open PBU settings by clicking S then navigate to the Misc. tab. In the Advanced Options section click Edit Custom Buttons:
2. In the pop up window click open file on the right for each of the buttons you would like replaced. The buttons can be any image file format and should be rectangular in shape or other shapes with transparency. You can find more buttons to customize by clicking the blue buttons at the top. The buttons have a normal state and a pressed state. Note: If you don't make a pressed button graphic your button will not change when pressed. Select Clear All Buttons to remove added files from all buttons.
3.The default PBU buttons are 224 x 68 pixels, and new buttons will be automatically resized to these dimensions when the main button scale is 100%. This scale can be adjusted at the bottom of the Edit Custom Buttons window to create bigger or smaller buttons:
4.Click OK twice and then you're done! Enjoy your new buttons!