Open a color clip by going to Open Other or by using the shortcut keys Ctrl+Shift+O. Choose Color. In the window you’ll see it says “transparent”. Click the button to the left of it that says Color. Choose white then press the Okay button. Shotcut will open a color clip in the video player under the Source tab.
Bring that color clip to your timeline and place it on v1 and extend it for as long as you want your video to last. Place all of your pictures on video tracks above v1. Now, you will have a white background for your video. 