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convert  one image format to another
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$ convert a.pdf a.png   #convert pdf to png

can also resize

$ convert image.jpg -resize dimensions newimage.jpg

dimension 
  e.g. 800x600 
       800x600! (exact)
       800 (preserve aspect ratio by letting program determine width)
     
$ convert image.jpg -rotate 90 newimage.jpg

See
https://www.lifewire.com/convert-linux-command-unix-command-4097060