No such thing as a dumb question :)

First and most importantly, Pixel Art must be saved in .png or .gif formats (Don't save as .gif in MS Paint though, it does odd things), as jpeg screws up colours and everything irreversibly and other formats aren't very web-friendly.

Pixel Art, at least in Pixel Joint's definition, is where control over every colour and single pixel placement (This doesn't mean you can't use floodfill or line tools though) is achieved. Any tools or filters which use automatic anti-aliasing, partial transparency/opacity and automatic gradients are Non-Pixel Art (NPA) according to PJ.

External Anti-Aliasing (like on that piece) are no-nos when it comes to a transparent background, i.e. when one can change theme like on here. It was probably originally tailored for a white background. AA is used for smoothing lines and the difference between colours, basically. The piece you posted has either been resized or jpegged at some point, so where it may have been PA once it isn't anymore.

Here's something I made for someone showing a .png, .jpg and weird paint .gif:

It's AA (or scaling interpolation)...against bright colors. Somehow the image ended up with an unsuitable (dark) background.

Edited by DawnBringer - 23 November 2010 at 6:45pm