Ok, this should be fairly painless :)
I see you have the NetPBM binaries already at the URL 
http://www.bitunde.com/netpbm/Make sure that all those files are "chmod" 755 so they can be executed. By default, they will likely be chmod 644 after uploading, which won't work.
I don't know your actual path, but as was said, should be something like /home/user/public_html/netpbm
You'll need both that path & the URL for Gallery install.
If you can't get NetPBM going, try uing GD2 for image processor, you'll have it, but is not as good as NetPBM/ImageMagick.
Here's the BIG Install FAQ on Gallery from XoopsGallery/xcgal (which I assume you are using)
-------------
Installation - Fresh Install:
-------------
To install XoopsGallery for the first time in your XOOPS system:
1) Unzip the package on your hard drive (Somewhere that you will remember)
2) Use a FTP program to upload the xoopsgallery folder and its contents to 
{Xoopsroot}/modules/ folder on your server (should be NO need to make a new 
folder called xoopsgallery. But if you do, just copy the contents of the folder 
and not the folder with its contents).
3) Set permissions on xoopsgallery/cache/tmp and xoopsgallery/cache/albums 
directories to 0777 (needs to be RW for webserver account, you might be able to 
reduce those permissions accordingly).
4) Login to XOOPS as an admin and click on 'Administration Menu' in user menu.
5) Once you are logged in as admin and in XOOPS site admin, Hover the mouse 
cursor over the 'system Admin' icon and then click on the 'Modules' link.
6) You should have arrived at 'Modules Administration' now, look down the module 
column list until you find the Gallery icon and click on it. XOOPS will now 
install the module and then prompt you to return 'back to Module Administration 
page' and then is the install complete.
7) Hover the mouse over the Gallery icon and click Preferences in the pop-up menu.
8) Important! Configure Gallery for your server. An important setting is the 
Graphis Suite options (Path to NetPBM / Path to ImageMagick / Graphics Suite). 
See the next section for details on that.
-------------
Graphics Suite:
-------------
You need to know the path to your NetPBM *or* ImageMagick suite is located, and 
configure XoopsGallery accordingly. You can check the settings to be sure it is 
working properly by accessing "Check NetPBM" or "Check ImageMagick" from your 
admin menu. 
Pesonally, I am and always have used NetPBM for no particular reason other than 
it was the first one listed in the options. You only need one of these suites 
configured for XoopsGallery to work.
If you have trouble with xoopsgallery, specifically "Problem creating 
thumbnails" or similar, see the troubleshooting pages on the Gallery website.
http://gallery.menalto.com/modules.php?op=modload&name=GalleryDocs&file=index&page=gallery1-install.faq.c.php#gallery1-install.faq.c.2Hope some of that helped. 

Personally, I find myAlbum-P easier to use, although I've used Menalto Gallery since early versions. Heh, I had a Gallery-based website that got about 2 million hits a month.
Here's myAlbum-P: 
http://www.peak.ne.jp/xoops/modules/myalbum/GL,
CaFF