Nokia :: 5230 Wallpaper Spontaneously Reverts To Default

I decided to replace the default wallpaper with something more appealing and so took a picture with the phone's camera and set it as the wallpaper. Later on, when I came to use the phone again I found it had reverted to the default wallpaper. The phone had not been switched off meantime. I reset the wallpaper again and, again, later on, I found it had reverted again. I store my camera images on the memory card (Sandisk 8GB) and have had no problems with any other data stored on the card. However, just in case the phone was having sporadic problems not being able instantly to read the card and so thinking it wasn't there, I copied the image to the phone's memory. I'm now waiting to see if it happens again. I can't upgrade the software to the latest release (R12) as I am on Tesco Mobile and their latest customised version of S60 is still based on R11.