Those of us who don't have widescreen computer screens are in for a treat with this Lucario wallpaper. An oft-neglected wallpaper size near perfect lineart, superb shading, and an interesting use of texture and glow provide us with an option we shouldn't have to stretch.
The lineart of Lucario here is excellent, with great proportions, an interesting pose that's not far off canon but still unique, and mostly smooth lines. The fur yellow details are particularly interesting. There are a few places, however, where the curve of the lines seems a little off. There seems to be a fairly sharp point at the back of Lucario's head. That trait exists in some official artwork, but not quite so dramatic; it could stand to be rounded a bit more. There is a spot on the nose, where the black and blue meet, that seems to be curved a bit too dramatically, but only by a few pixels at most. I also think the angle of Lucario's right back claw (viewer's left) seems a bit off, but I only noticed it after looking for several minutes.
If I may, the placement of the signature is less than ideal from an artistic perspective. I understand that helps prevent thievery, but your watermark or signature detracting from the artwork is a risk you take. I really can't see anywhere on Lucario himself that that signature would go better.
The pose and facial expression are what really make this piece unique. Very often, art of Lucario has a very grim, serious look to it. This is mostly due to fans being influenced by Lucario's portrayal in one of the anime movies. However, this captures a more innocent, doglike, even animalistic moment for Lucario without robbing him of the nobility he has in that movie. This Lucario is hot blooded and ready for battle, without having any bloodlust or delusions of human sophistication. Agnes has turned Lucario into a common Pokemon you could feel ok giving to an OC again, rather than a psuedo-legendary who must stay unique and unapproachable.
The texture on the lineart version of Lucario is very nice, and the effect of it is to give us a more complete picture. The lack of colors makes the whole thing muted, like an after-echo of the colored Lucario's passion. It elicits the idea of the rapidly fading memory of an exciting battle. It isn't the most original wallpaper trope, but it certainly works better here than many of the other places it's been used.
The addition of Lucario's name is a mixed bag; it does detract somewhat from the raw emotion of the picture, but it also lends an almost pokedex feel to the piece. The choice of font suggests Japanese script at first before resolving into a readable italic one; This is visually interesting and adds a layer of coolness. I find that the use of red in the text overpowers the actual image of Lucario slightly, but I'm not certain that any remaining color in the piece, particularly pale yellow or light blue, would improve this, and I'm not certain a monotone white font would be interesting enough. Basically, the way it is now isn't bad... I'm just not sure it's good, either. Still, if your biggest problem in a piece is a font color choice when the text is there for visual interest and not for readability, you're doing pretty well.
This wallpaper really helps me understand what other people see in Lucario as it appears in the games rather than the Lucario of the anime. It's cute, it's passionate, it doesn't detract from the toughness or awesomeness of the Pokemon, and with one tiny exception the colors outside of the full-color are used very well. The lineart is nearly perfect and arguably more attractive than many instances of official artwork. The piece has a depth of both visual appeal and character that makes it pleasant to look at. This works as a piece of art as much as it does as a wallpaper; that's saying something.