Error message frozen on screen

[AC Question]

I was trying to download photos from a link sent to me by SMS (reputable Google photo file). During the download of the photos - not many, only approximately 15 - a white error box came up on the screen. It reads UNFORTUNATELY THE PROCESS ANDROID.PROCESS.MEDIA HAS STOPED there is an OK button. I have pressed the OK button repeatedly but I can't get rid of this message which is freezing the phone and not allowing me to do anything including answering incoming calls. I have been to the Telstra shop and the technicians have looked at it and can't fix it. I have turned the phone off and on again 4 times. I have taken the battery out and put it back in again. I need to stop the application running but can't find anything like the PC option of Task Manager. I have tried to get into Toolbox and Application Manager in the phone but can't. I believe it it stuck in an unending download loop, trying to download these photos but unable to. How do I quit this application?