With the sprites, the effects kind of make them hard to judge. Never, EVER save sprites as JPG. As people before have said, always save as PNG, as JPG leaves little, what I call 'bits' around the image and distort the colours. Also, never re-size your sprites. When you work on them, zoom in to see what your doing, and keep them the original size. Sprites need to be clean and you should be able to see the original intended image exactly.
Onto the sprites themselves. The Spinda/Minun has barely any shading at all (from what I can see), apart from the shadows on the ears and a little on the body. There's a bunch of tutorials on these forums that help in fusions and recolouring, so you should perhaps brush up on those first. The Pikachu/Minun has a weird flat head, as Pink Skitty mentioned, you need to round it off a little so that Minun's ears flow with Pikachu's body better.
I think you should read tutorials found
here, as I'm 99.9% sure you'll improve that way.
EDIT: Bah, I just realised I've mostly repeated what people before have said, sorry about that. ._.