You could roll out a piece of fondant or gumpaste to the size of the picture, Let it dry and attach the image to the sugar with royal icing or piping gel. I would not put it on colored icing it will change the way the picture looked but if you put that piece of fondant/gumpaste behind the picture than it would be fine on the shamrock. Keep your images out of direct light and careful moving them around alot, they can fade and crack or even curl up so it’s not something I would add to the cake till the day of. Do a little research on it just to be safe. I have used them in grocery store bakeries but it’s been awhile. ALso start checking where you can order them cause last time I did one I had to order it online, it didn’t take long but alot of grocery stores wont print images for you. Hope that helps!