It's not a simple procedure if you aren't familiar with the directory structures inside Wing Commander's compressed data archives, these are your .TRE files.
For both games, you'll need a utility called TREMAN to unzip the game data (from the TRE files on the disks) and make the changes you require, that is the tip of the iceberg. Once you've made the changes (which you'll require other applications to do) it won't be as simple as simply zipping the TRE files back up and starting again, you'll need to find how to tell the games to look at your modified files - for Secret Ops this is quite straight forward, but I've no idea about WC4 or Prophecy, sorry.
No problem, and belated welcome to the CIC. If you need TREMAN, simply PM me with your email address and I'll send you the (only) working copy I could find, I can also provide a few tips on how to execute TREMAN and which files you'll need to extract, have no idea how to edit the artwork but it's simple enough to extract and convert to .BMP with a couple of apps.