Yes, I bookmarked it just the other day. Imagemagik... it can be located
from synapic manager.

see these two websites for some examples of what to use
www.smokinglinux.com/.../howto-batch-image-resize-on-linux
www.ibm.com/developerworks/linux/library/l-graf/

That second page has some other useful tools that I haven't used before.