Couple quick things.
Try not to use verbs that end in "ing" in your scripts. Instead of having Sam and Sue walking to the living room simply have them walk. Scripts are written in the present tense and having the ing infers past tense slightly. Also it's a early warning sign you may not be aware of proper screenwriting techniques. Any chance you give the reader to bail early they will. Not picking on you, I did this all the time when I first started. Everybody does.
If they are singing Happy Birthday that needs to be written as dialogue.
If the scene changes to the living Room you can put in a sub slug (I can't remember the technical name for it right now, sorry, spacing) Just write LIVING ROOM in place of a normal slug line to show the transition.
You don't need to describe Sue as short and then give us her actual height. Unless that specific height is crucial to your story then you don't really need to include it. And even then, 4'11 is unneeded. Would really limit your casting options too.
Ignore using "we see". Just describe what's going on.
The flashback was funny.
There are many ways to write a flashback. The key is to just be consistent with them. If you are starting one you need to end it. http://screenplaywritenow.com/format-screenplay/
The photos would be a montage I'm guessing???? In that case you need to describe each photo being shown. Would be a good way to build character with descriptive photos by the way. Or maybe you pictured a bunch of photos being thrown onto a table. A photo album? You need to be specific when you write. Think of what you imagine on screen and write exactly that.
After the photos we are in the present again? Where the photos a flashback? Again, if you are not accurate in exactly how your movie works it will end up a mess.
I'm at work so my time is limited. You have some ideas for sure and congrats on getting a full length script done. But there's a lot of things to fix in these first two pages. Besides the formatting stuff a lot of the dialogue is too on the nose. Mitch's voice overs are just an exposition fest. I understand the point of them but the execution needs work. Best of luck on this going forward.