Jeez I sound like an advert for Access 2007 today.
(i.e. it was 2007 that introduced that ability).
In earlier versions you'll need to fake it. Since having an image which includes text too (the standard workaround) isn't an option for you as you're neding to change the text trivially on the fly - you can always use a couple of controls, a label and an image control laid out on the form with a transparent command button over the top of them (I'm afraid visible but transparent background command buttons are also new to 2007).
Clicking them will actually raise the command button's event - but you'll see the label and image control.
If you wanted the appearance of a click depress you'd have to code that.
For an example of just simply using labels as command buttons - see the demo in the examples page linked to in my sig below (Command Label).