Batch process images plugin for Gimp...

To install

sudo apt-get install gimp-plugin-registry

If you prefer GUI installer, open Synaptic, search for gimp batch and mark for installation

Synaptic

To use the plugin, open Gimp and go to Filters -> Batch -> Batch Process... menu

GIMP batch process plugin

Add files to be process at Input tab

Input tab

Set value to whichever tab processes that you need and set the image type at Output tab (default is .bmp).

Resize tab

Click Start button.

Note:
Tested with GIMP v 2.6.10 on Linux Mint 64-bit.