What version of PB are you using? I'm not incredibly up-to-date on when changes were made to PB, but 10.5 does highlight buttons (both picture and standard text), as well as objects like radio buttons, check boxes, and even drop downs...
I'm just spit-ballin' here, but if you're using an older version of PB, then this may be a clunky method of doing this, but I did use it in my Sr. Project a few years back and it worked flawlessly... Create a label behind your picture button. In the mousemove event for your window, store the X1, X2, Y1, Y2 values of your button, then do the simple logic of getting the pointer X,Y, and if it's withing the bounds of the button, then set the backcolor of the blank label to something other than its default. Creates a "halo" effect around an object that is variable in size/color/etc.