Photos app - Photo not showing to change user picture / Videos not showing on imovie

Hi

I am having a lot of difficulty using Photos... I am more used to using iPhoto however, having recently reformatted my macbook after some serious lagging issues (went down to apple support and they told me it will help to resolve the lag issue), Photos have been driving me crazy!

I am unable to change my User login photo because there is no photos showing from Photos... and when I use iMovie, my videos and photos within Photos do not show up too. How do I got about resolving this? I have been searching the forums for an answer but it has not been working...

P.S. If there is a solution on another thread, could you please kindly post the URL? My apologies if I am duplicating a thread with a similar problem.

Additional details of my macbook....