Answer

As of version 2.3 of Movable Type, an alternate method of creating thumbnails is supported, using the NetPBM suite of tools. If your system has NetPBM but not Image::Magick, you'll need to do the following:

Install Perl Module

Install the IPC::Run Perl module, which Movable Type uses to talk to the NetPBM tools. Download this file from Six Apart.

On your FTP server, create a new folder called IPC within your extlib folder. Then upload the Run.pm file and the Run folder from the distribution into the IPC folder on your server.

Configure Movable Type

Edit mt-config.cgi¹, and add this line:

ImageDriver NetPBM

By default, Movable Type will look for the NetPBM tools in one of the following locations: /usr/local/netpbm/bin, /usr/local/bin, or /usr/bin. If you need to set a different path, add another line to mt-config.cgi:

NetPBMPath /path/to/netpbm

Save (and re-upload, if necessary, in ASCII mode) your mt-config.cgi file, and you're done.

Other

If your host does not have NetPBM, or if you run your own server, you can download and install the NetPBM suite of tools. If you'd prefer (or need) the precompiled binaries, the Gallery developers provide a set of binaries.

An alternative tutorial for installing NetPBM is available at Sniptools: Thumbnailing with NetPBM and Movable Type.

¹ In versions prior to 3.2, the configuration file is named mt.cfg.