(I check these links every once in a while. Still, a link that's here may have become outdated. If you find one, please tell me - preferrably with the new location.)

http://www.libtiff.org/tiff-v3.5.4.zip

This is where you can get the latest version of libtiff tested with paintlib. libtiff is nessesary for tiff support with paintlib. There may be newer versions of libtiff than this one available at http://www.libtiff.org, but this is the version currently used to build paintlib.

LIBJPEG ver 6b

You can download the independent JPEG group's library here. Note that the paintlib project files are made for ver. 6b of the library. The ijg's site is at http://www.ijg.org.

PNG information The PNG home page. The official file format specification can be found at http://www.w3.org/pub/WWW/TR/REC-png.html. libpng itself is at http://www.freesoftware.com/pub/png/src/lpng108.zip. You also need zlib for png support (http://www.info-zip.org/pub/infozip/zlib/zlib.zip). The zlib home page is at http://www.freesoftware.com/pub/infozip/zlib/.
libungif Download the gif decoder library here. The main libungif page is at http://prtr-13.ucsc.edu/~badger/software/libungif/.
NETPBM Link to a NETPBM ftp site. NETPBM (based on PBMPLUS) is a package of image file decoders and encoders including source code. Sadly, the last version of this package dates march '94. NETPBM supports a lot of formats and may be a good starting point if you need to write your own decoder. There are a lot of other places where NETPBM can be found. Do an archie search to find the location nearest to you.
Graphics File Formats book and FAQ Not a net link, but a book recommendation:

Encyclopedia of Graphics File Formats: Complete Reference with Links to Internet Resources, by James D. Murray & William van Ryper. 2nd Edition March 1996. 1050 pages (est.). Includes CD-ROM, ISBN: 1-56592-161-5, $79.95 (est.)

This book/CD combination was my main source of graphics file format information whenever I couldn't find code. James D. Murray also maintains the graphics file format FAQ regularly posted to comp.graphics.misc.
Graphics File Formats home page This page contains updates for the info in the book mentioned above. It's usable if you don't have the book, but of course they refer to it a lot since they want you to buy it...