As the name suggests, BIR (Batch Image Resizer) is an open-source, cross-platform, batch image resizer which does exactly what it says.
The latest version available is BIR 2.0, which has been released a while ago.
The latest Batch Image Resizer builds are available via some third party PPA, so installing the software on Ubuntu and derivative systems should not cause too many problems. You just need to add the PPA to your system, update the local repository index and install the birdfont package:
$ sudo add-apt-repository ppa:dhor/myway
$ sudo apt-get update
$ sudo apt-get install bir
Optional, to remove bir, do:
$ sudo apt-get remove bir
The installation instructions should work on Ubuntu 15.04 Vivid Vervet, Ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr and derivative systems like Linux Mint 17.2 Rafaela, Linux Mint 17.1 Rebecca, Linux Mint 17 Qiana, Pinguy OS 14.04, Elementary OS 0.3 Freya, Deepin 2014, Peppermint 6, Peppermint 5, LXLE 14.04 and Linux Lite 2.