I can't recommend any code library, but it's easy to use GhostScript to convert PDF into bitmap formats. I've personally used the script below (which also uses the netpbm utilties) to convert the first page of a PDF into a JPEG thumbnail:
#!/bin/sh
/opt/local/bin/gs -q -dLastPage=1 -dNOPAUSE -dBATCH -dSAFER -r300 \
-sDEVICE=pnmraw -sOutputFile=- $* |
pnmcrop |
pnmscale -width 240 |
cjpeg
You can use -sDEVICE=tiff...
to get direct TIFF output in various TIFF sub-formats from GhostScript.