You can choose between several preconfigured navigation buttons using the imageNavStyle configuration option. Possible values are: CURSOR, BIG, CIRCLE, SQUARE, CLASSIC and TRIANGLE.
However, SimpleViewer-Pro does not come with source code and it is not possible to replace SimpleViewer's own icons with your own custom images within a gallery.
An alternative might be to create your own buttons (outside the gallery) and have them act like the gallery's own navigation buttons using the SimpleViewer-Pro API (specifically the showNextImage() and showPreviousImage() methods.
To see an example of this in action, please see the online demo here. You could view the source of the page in a browser, copy the code you need and replace the 'Show Next Image' and 'Show Previous Image' buttons with your own custom images.
SimpleViewer Support Team