Minecraft wiki entry for Experience, associated text: The first picture of the Orb. Notice, it is on the very left side of the page not the torch in the middle.
Notch later uploaded another version of the image, claiming the Orb was a "small error".
Orb in the first picture, enlarged.
The Beta 1.7 "change-list" (the Adventure Update was supposed to be Beta 1.7).
The decoded "change-list".
Different sized experience orbs dropped by a skeleton upon death.
Raw file of the experience orb texture. Note the white sections, which are tinted accordingly green/yellow, the orange sections, which are tinted to a lesser degree, and the five unused orb sprites beyond size 11.