Converting different graphic formats

• One way is to open the file with an almost almighty tool like GIMP and save it in the desired format.
  
• Another option is to use the command line tool convert. To create a Post Script file out of a JPEG image just enter the command convert inputfile.jpg outputfile.ps - of course with the correct file names.
• More options like resizing etc. are specified in the corresponding documentation, so just type man convert.