Start with Original PSD/AI/etc. or High Resolution PNG or JPEG

• If you contract out: get the original/source files!
  Keep track of them!
  Make sure others know!
• Always start with the original file format (e.g. PSD, AI, etc.) or high-resolution PNG or JPEG.
• Size images accordingly!
  • Don't shrink full-resolution images by using "height" and "width" CSS
  • Don't enlarge images from smaller ones -- you can only go one way (larger to smaller)