Not without modifying core.  All of these buttons are ASP.NET linkbutton controls that use the CSS style CommandButton, which you could simply modify for your own tastes in Admin -> Site Settings -> Stylesheets.  Unfortunately, there's no way to know whether you're modifying an Update, Delete, Cancel, etc. linkbutton.

One thing you could consider is to modify core and do this on demand by replacing the Linkbutton control with your own.  I don't recommend modifying core, and to be honest this approach is overkill.

The way I would do this, if I had to ... was to write some JavaScript that iterates the DOM looking for anchor tags with an ID that ends with cmdUpdate, etc.  Then add in the necessary image as needed.  It would certainly work, but you'd want to make sure it didn't go awry in AJAX scenarios.