On linux and Mac I use ImageMagic or ghostscript (programs like Imagemagik and Gimp use Ghostscirpt in the background anyway).
Just this second use GS to batch concert some PDFs to jpegs:
gs -dNOPAUSE -sDEVICE=jpeg -dJPEGQ=100 -r400 -sOutputFile=card%d.jpg *.pdf
-sDEVICE specifies jpeg output
-dJPEGQ=100 is the jpeg quality (0-100)
-r400 is DPI, useful for when converting a vector like a PDF/PS
The *.pdf is the input file, basically all my PDFs, *.jpg will convert all jpegs etc.
To resize an image add the -g flag, -g1024x768 will scale the image to 1024x768 etc.