Glyphicons are created using the content property and the :before pesudo class and they don’t apply to replaced elements such as images or inputs (there is no before or after to an image and an input doesn’t really have any content apart from that supplied in its value attribute).
You need to use the button element as mentioned in that other thread.
EDIT:
I’m not sure if this is well supported but you could try putting the hex value (") into the value attribute.