I want to make a thunar custom action that would reduce an image to 1024x768.
This works from the command line.
convert -resize 1024X768 smaller.jpg small.jpg
I have this so far.
convert -resize 1024X768 %f
The command you found and cited above uses the convert app which is part of the imagemagick package.
To add that to Thunar, launch Thunar, do Edit , then click Configure custom actions… to go to the Custom Actions Manager, where you can create, edit or delete your custom actions. Enter a new custom action of convert -resize 1024X768 %n.jpg %n.jpeg
then click OK to save the export to the same filename ending in .JPEG .