I may be way off base with this, but have you tried "Simple Image
Resizer"? To find it in Synaptic (etc.) you need to add a PPA to your
Third Party sources, as below, and refresh. It can then be found in
the file listings by the name above. It works very well for multiple
files for me and there are many ways to set it up.

One caution, the default file type is "bw" (black and white?), which
is probably not the one you are working with but there are many, many
other formats in the dropbox. You can choose files or directories and
select the ones you need to resize. The output directory can be
selected and also a prefix to distinguish the compressed files from
the originals. The filename, in case you want to start from the CLI,
is sir.

deb http://ppa.launchpad.net/rsachetto/ppa/ubuntu lucid main (replace
lucid with your version name)