There are three sources of artwork and photographs for your computer. You can also draw your own material in software such as CorelDraw or Dabbler. If you need photographs or drawings on a regular basis, then consider purchasing a scanner. These come in two sorts - the hand scanner and the more expensive flatbed type. You can't save scans in vector format - they can only cope with gradient fills.
Bitmapped graphics are much more capable when it comes to subtle shading or colours.
Be aware that Mac and PC machines don ' t use TIFF in quite the same way . Now for the technical bit. The front cover graphic is a huge 12Mb without anything else. The only material that works well in bitmapped format is straight lines, because everything else has to be approximated by steps. Vector graphics seem to have everything going for them.