Have a peek in Volcanic and see where the .jpg's are located. If that fails then just try and find them manually (search).
To start up Volcanic from the FreeSO Launcher, just right-click the "Play" button and confirm.
If you are not using the launcher then go to "C:\Program Files\Maxis\FreeSO" and start Volcanic.exe
NOTE: If you are changing texture(s) for any floor tile(s) only you will be able to see the custom textures/objects; other people will need to download the textures/objects that have been changed.