You need to install imagemagick. Here is a tutorial on how to use it:
www.ibm.com/developerworks/linux/library/l-graf/

I have not used it,
not being a CLI fan.

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.

I didn't even think about that.
I was writing a php script using imagemagik but forget about using it
all on its own.

That saves you the trouble of having to install and learn a new commands.

to resize images, you can try this imaging sdk , it can be applied in c#, vb, asp, etc. and the quality is also quite good ,you can have a try.
www.rasteredge.com/.../

If you want to adjust image size to suit their needs, this image resize component may be a good choice. It provides flexible sizing options to control document & images in different formats or in bitonal, color and grayscale. Here is the link: www.yiigo.com/.../how-to-resize-image.shtml