You can choose image button and then choose from the available image buttons, or click on the folder icon when you select the image button and choose from the Gallery of Captivate 6.
Few image buttons change color on hover or onclicked, these effects are predefined in them.
Or you can edit that button on which you are working, from properties Panel.
The Post and Cancel button on this thread seems like a transparent button in Captivate with different fill and stroke color, and transparency is also less, and with corner radius of 0.
You will find these all optionns, when you will click on that button and then click on the properties panel.
Thanks.