How do I convert images to whatever icon MAc uses?

Question

Client: Mac OS 10.6.6
Hi All,
I am coming from the Linux side. In Linux, if I an find an Image I like, I can use it on my desktop Icons. Format is not an issue.
Questions:
1) in Mac, I have found it wants a particular format. How do I convert Images (jpeg, png, bitmap, tiff, etc.) to that format?
2) once I have the image in place, do I need to keep the image around or is it copied into the icon's hidden (in Finder) application directory?
Many thanks,
-T

Answer 1

For anybody using MacPorts, there is a port called makeicns that creates the needed .icns files from the command line.  Usage is pretty simple (from the command):
makeicns v1.4.10 (284bd686824f)
Usage: makeicns [k1=v1] [k2=v2] ...
Keys and values include:
    512: Name of input image for 512x512 variant of icon
    256: Name of input image for 256x256 variant of icon
    128: Name of input image for 128x128 variant of icon
     32: Name of input image for 32x32 variant of icon
     16: Name of input image for 16x16 variant of icon
     in: Name of input image for all variants not having an explicit name
    out: Name of output file, defaults to first nonempty input name,
         but with icns extension
  align: [center, left, right, top, bottom] {First letter suffices!}
Examples:
  makeicns -512 image.png -32 image.png
      Creates image.icns with only a 512x512 and a 32x32 variant.
  makeicns -in myfile.jpg -32 otherfile.png -out outfile.icns
      Creates outfile.icns with sizes 512, 256, 128, and 16 containing data
      from myfile.jpg and with size 32 containing data from otherfile.png.