ImageMagick error message ???

[Jim]

Fedora 17 / KDE 4.9
Having trouble converting density of a Image tom.jpg.
This error message below , is it because a empty bob.jpg image is not 
there ?
How do I fix this problem?

$ convert -density 72x72 tom.jpg bob.jpg
convert: delegate failed `"html2ps" -U -o "%o" "%i"' @ 
error/delegate.c/InvokeDelegate/1058.
convert: no decode delegate for this image format `/tmp/magick-3XBbxcw8' 
@ error/constitute.c/ReadImage/566.
convert: missing an image filename `bob.jpg' @ 
error/convert.c/ConvertImageCommand/3016.