I just called a representative at Straight Talk about getting the upgrade to Jelly Bean for my LG Optimus Showtime. He said there was a way to upgrade it. He then proceeded to lead me through the steps of going to the menu, hitting system settings, scrolling all the way down to "about phone" and clicking software update and making sure that the automatic update box was checked. However, when I went back to "software information" under "about phone" it still says I'm using version 4.0.4. (Ice Cream Sandwich). So apparently there is no upgrade for Jelly Bean as of late.
No news yet if the LG Optimus Showtime will get a Jelly Bean update. But someone from the Straight Talk forum (perhaps a Straight Talk employee), that the Optimus Showtime is not getting a Jelly Bean update. In this case, your only hope is to install a custom ROM, given that there is one available.