Hi, it sounds like one of two things, your frog had been injured and has got an infection or fungal growth in the cut or it has a tumor or abscess. A walnut is quite a large size to have on a frog is it causing it much discomfort? is it behaving normally? the only problem with taking it to the vet is they arnt great under anesthetic. I doubt there is much else you can do apart from physically removing it via the vet. To post the picture go to
Gallery on the blue bar at the top of the screen, click on upload photos in the green bar in the top left, image library, and make sure you image is about 1000 by 800 pixels in size.