What is the best method to compress jpegs to conserve iPod storage space?
Probably nothing, if you want to leave them at the same quality. The JPEG/JFIF format is very efficient unto itself and external lossless compression utilities are not likely to have much of an impact.
Your best bet, is probably to load your JPEGs into an editor and re-save them at a lower quality setting, or resample them down to a lower resolution, but then you’re lowering the quality of your images. May not be a problem if you’re only going to look at them on your iPod screen, though.
leachim is right. I think the best bet would be to re-size them down to the size of your iPod screen resolution. That will probably make a huge difference, but if that’s not enough of a reduction, you can reduce the image quality in the JPEG settings.
There are many free tools out there to do image resizing in a batch mode. I think Irfanview can do this.
I believe iTunes already compresses JPEG files to more closely match the resolution of your iPod/Phone screen, they call it “image optimization.” In fact here is a person wanting to turn that very feature off: How to turn off iTunes photo "optimizatio… - Apple Community
Now if you take the photo on your iPod/Phone, it will probably remain full resolution on the device.