Message notification problem/SMS come through as MMS

First I have a Strighttalk LG Optimus Showtime.

I have a friend who whenever she sends a regular SMS I always receive it as a picture and one of these times I tried opening it and it deleted itself but the notification is still there. I can't find the manage applications under my settings to clear out messages as others said to do, I also shut my phone down, tried installing a new texting app all with no luck. If there are any ideas it would be much appreciated