"Extra output images" needs to be enabled in the GUI (or .tgd file). The -ox option on the command line should then override the path and filename of those files. Make sure that 'IMAGETYPE' is included somewhere in the filename.
If you use -o, Terragen should acknowledge this by printing a line to the console beginning with the text "Output filename (-o) = " followed by the path and filename you specified.
Likewise, if you use -ox, Terragen should acknowledge this by printing a line beginning with the text "Extra output filename (-ox) = " followed by the path and filename you specified. These lines are printed after the project has loaded, but possibly before other resources are loaded, and before the main rendering begins.
Can you give me an example of the full command line you're using?
Matt