You'll laugh at me because I actually looked in my "Collector's Value Guide of Pokémon Trading Cards" to check Pikachu out x)
I think you should make the eyes further apart, the nose smaller and his belly a bit rounder, if I refer to to the Gym Leader's Pack, Lt. Surge's Pikachu LOL (it's worth 4.25$ dontcha know).
Maybe an opposite light source with a different colour could be interesting, and perhaps a colder background colour could make Pikachu stand out more.
I'm pretty much amazed you did that in Inkscape, but then such a vectors program has a lot to offer.
As for the Link wallpaper, you have anatomical issues: the thighs are very long, and Link's shoulders aren't very broad (even though he's a child there, but I'll explain) even for a boy, and for now it looks like woman proportions (narrow shoulders and a wide pelvis). The clothes seem to float around Link's body, so I suggest you think about how the clothes would fall and fold on someone's body (or you could use a LOZ64 artwork of Young Link as a ref, that's well done). The light source is ambient, which is a shame
. The eyeliner around Link's eyes is something I don't like, because it makes him a bit androgynous :/. I think you could make the sword's handle have more definition, because it looks like a continuation of the guard.
Anyway... I hope I was of help and looking forward to more stuff!