So I have worked out a process that I am going through now. This process does the following:
1 - Pulls down the images from the skyview nasa site for each NGC object and combines them into an RGB color image.
2 - Transforms the image from a jpg to png format picture
3 - Renames the objects to the thumb-ngcXXX.png name format they expect.
I am in the middle of pulling the first 1000 NGC objects now. I will delete the duplicates that are already included in the kstars images. I will also have to re-grab several of the objects with a larger pixel size to fit the full image into the resulting frame. I am only going to do that for images that don't comfortably fit in the 100x100 pixel frames I pulled during the initial run. If this all goes well, I am pretty sure I can do this same process for the other catalogs as well. If knro is interested, I will upload the files for inclusion in the distribution.
If you want to do this process for yourself, you can download a few packages and do this yourself.
1 - Download Skyview in a Jar from the NASA website
2 - Install mogrify and mmv packages in linux
3 - use the following syntax on each NGC object you wish to grab the file from the skyview website for:
java -jar skyview.jar position='NGC 1' survey=dss,dss2b,dss2r rgb scaling=linear pixels=100 output=ngc1
NOTE: You will need to make adjustments for the NGC name and the pixel size of the image.
4 - Transform the image from jpg to png with mogrify
5 - Rename the files with a batch rename in mmv or individually if you want to lose your mind.
6 - When you have the objects you want to include thumbnails for, you can copy the png files to $HOME/.local/share/kstars and it should show up if the objects are named properly and are of the correct format.