If you want your images to look good, do not simply scale up/down a raster image.
Even with a vector based source, this doesn't always work so well, though the results are usually passable.
The best example of this I can give is this: say you wanted to resize this 48x48 icon into a 24x24 alternative. In 24 pixels, you likely would want to reduce the faux-writing lines from 4 down to 2 or 3. If you just scaled down the image and left it with 4, things would get messy and cramped.
Ultimately, it depends on what the images are being used for.
Static images such as buttons and icons you'd want to manually resize, to make sure every pixel is perfect, and it communicates the same concept.
For animated images, the eye doesn't get enough time to fully scrutinize it, so flaws are more acceptable.