It's not that hard to extract the data files.
Just run extractpak
in the Monkey Island SE directory (where the Monkey1.pak is).
This will extract all artwork and store it in the "DDS" format.
Now you just need a tool to load and save DDS files.
There seems to be a Photoshop DDS Plugin by Nvidia
so you might want to check it out if you use Photoshop.
That's actually all about it. If you finished your work just convert it back to the original dxt format with dds2dxt