In the following tutorial Paul Yanez shows how to create your own custom button with images, gradients and transitions.
1- import an image and place on the artboard
2- add a rectangle with a radial gradient the same width and height as the image
3- add a rectangle with no fill and grey color as the border
4- select all 3 objects, right click and group into a canvas
5- right click and select make into usercontrol
6- select button as the type of usercontrol you want to create
7- right click and select “edit template/edit current”
8- select the states panel and go to the “mouseover” state and set the gradient’s opacity property to “0″ and change the stroke color of the rectangle from grey to white
9- change the transition duration to 0.3 sec and set the transition to “cubic in”