.png and .gif work well for file formats. Some programs screw up the saving of .gifs, though and lose quality or distort colors, so I use .png.
Wow, yoe can't tell me you're using every color in that palette for the moniter. Look! I can hardly tell the difference between the two shades right next to the last one! Looks like you just selected two colors and made a gradient between them and thought "cool, I've got a palette." If that's what you did, I suggest hand picking each color. If you did hand pick each color, then I apologise for accusing you otherwise, but you really need to cut down on colors. Especially since everything has hard edges and doesn't really need a smooth transition.
Also, the shadow being cast by the moniter, see that little part of the shadow that overhangs the edge of the CPU? Well, it should be vertically stretched, since the surface changes in its slant. If the surface a shadow is cast on changes its...slant or rotation or steepness or what-have-you - anything that's not perpendicular to the lightsource - the shadow conforms to the new surface shape.