The best solution at the moment for a gallery for your PostNuke site is called My_eGallery. It is available for download at the Sourceforge.net My_eGallery page.
The installation of it can be completed in a few steps:
STEP 1 Download and unpack the zip file at your local hard drive.
STEP 2 Upload the content of the folder to the /modules folder of your Postnuke.
STEP 3 Go to the Administration area and then to the Modules section.
STEP 4 Initialize the module by clicking on the link next to it.
STEP 5 Activate the module and create a block for it as described in the Modules and Blocks section.
Well done! You have successfully installed the Gallery module to your PostNuke website!