How can I reduce the resolution and storage size of photos synced to my iPhone?
Unfortunately your iPhone already does this with a majority of your photos, and maintains a specialized cache for this purpose, so I doubt there will be any programs to help with this. You can see this via http://support.apple.com/kb/ts1314 

If you want to rescale your photos in an automated fashion, and you are using your macbook, you can easily create an automator action to create smaller images with entire folders of images. However it seems like you don't want to maintain a second set of lower resolution images.