I have also went into the RetroPie-Setup menu and went into the splashscreen area. I did a search, through my FTP client for the names of those files, and cannot locate them (thinking I could just throw my custom splashscreen in that directory and have it show up).
On top of that, I rebooted into Raspberian and used the LXTerminal to do the same thing. Executed the same line of code I used above, and it went just went to the next line with the blinking text indicator (which is normal if it works, correct?).
My apologies for my novice abilities with Linux and RetroPie, it has been a while since I used Linux on a regular basis, and this is my first week with RetroPie.