Sadly I don't have access to a device like yours for testing purposes but
avconv
has a good scaling filter.
avconv -i BigInputimage.jpg -s 640x480 640x480smalloutput.jpg
This should do the trick. For multiple files:
for i in *.jpg; do avconv -i "$i" -s 640x480 "out-$i.jpg" ; done
Where jpg is the extension of the images you wish to rescale.
multiple file command inspired by January answer regarding video here: How can I use avconv to convert multiple files in the terminal?
Based on what little I could find it doesn't look like the Ubuntu touch gallery app supports resize as an edit option. If this is incorrect,
please advise.
Since the terminal is available now and it seems cli tools can be run on ubuntu touch. This seems a valid answer.
To follow the Ubuntu Touch core apps development, you may find this page of interest: https://launchpad.net/ubuntu-phone-coreapps