Images not displaying in full during playback in sequence window

Hi

Apologies if this has been asked and answered a ton of times. I am making what should be a straightforward video for my daughters 13th birthday. I want to merge loads of photos with music. The problem is the photos all have different resolutions and aspect ratios, taken on various Iphones, androids and regular camera over the years. I also want to mix in some videos taken on varous devices. I have imported all but on playback lots of the images are not displaying in full, only partial. 

Is there a suquence setting that will uniformly adapt all the various resolutions and ratios so they all display in full while I edit the timeline? or do I first need to convert all images and videos to the same res/ratio?

If someone had an idiot proof guide to how I do this it would be much appreciated.

thanks.