Run in the BQ the command to get the information about the actual used device and channel (this formation is stored in the phone in the file /etc/system-image/channel.ini):
$ system-image-cli -i
current build number: 25
device name: krillin
channel: ubuntu-touch/stable/bq-aquaris.en
last update: 2015-09-13 07:21:14
version version: 25
version ubuntu: 20150825.1
version device: 20150821-736d127
version custom: 20150821-887-33-32-vivid
which contains a list of the files to be removed; than all other files are after removal just tar'ed in into the target file system which is mounted on .../system
The above files are the full images URL; You can find these and the delta file names manually by looking in the index.json files on for the relevant channel, in our case here exatly in (at the end, while writing this the delta between r24 and r25)