Within the button widget's properties, you can choose an icon. I think it would be a good idea to add some icons to the list. The icons that should be added should be the ones associated with playing music or video. Im talking about the (play) ► and (rewind/seek) ◄◄ and (fast forward/seek) ►► as well as the generic stop and play icons. To put these icons on buttons would be great!
However, to even put custom icons (i.e pictures) on the buttons as the icon would be epic!
The icons that we have right now are standard jQuery Mobile icons. You can add custom icons via CSS/JavaScript. I agree, having more custom icons is a good idea.
I really agree on the idea of having custom icons that can be uploaded specially for buttons and nav bar which sometimes make it very limited to choose icons specially if your functionality is not very close to any of the icons offered.....this could make make the programmers job quite more simple rather than making lots of lots of new icons because the user itself will provide the needed icons by himself....