No problem , glad I aint the only one who's out there glueing picture to guitars ! The "trim it so the binding shows" is a great idea ! Go for it - it shouldn't be a problem , although you'll have that paper-thick transition at the edge of your binding. Clear it smooth and you'll be fine. If youre doing a whole different color , by all means , remove what you need to. lol
And you can, can paint a guitar by scuffing up the old finish and going over that , but now you're getting kinda thick in the paint area . Thats entirely a personal choice , though.
Best of luck - post pix.