This might be a linguistic issue, as I didn't learn about the narrative structure of the flashback through writing from the anglophone world...
Oh, me neither, I'm not a native english speaker, so there very well could be some linguistic issue at play here. Let's see...
In spanish -my mother tongue- but I think also in other languages, when we want to avoid anglicisms like "flashback" and "flashforward" (and in fact also when studying this devices "scholarly") we refer to this kind of narrative resources as "analepsis" and "prolepsis", from greek. An analepsis can be as subtle, minimal and inconsequential as writing: "And today I am back here, to the city where I was born", and regarding the prolepsis...
Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
That's the first sentence of Gabriel García Márquez "One hundred years of solitude" and one of the most famous examples of prolepsis in spanish in the last century or so (and quite often used when explaining the concept, as also is the begining of "Chronicle of a death foretold" that directly spoils -in modern terms- you the end of the book). Hardly a fully interjected scene, just a passing mention, but one that strongly resonates.
So I guess you could say the polaroid scene isn't a
hard flashback in some sense or definition of the word, just Lena referencing or reminiscing past events, but I would argue it is in fact an analepsis (which, iirc, originally means "recovery", "retrospection"), one homodiegetic and completive to be more precise. It does in fact interrupt the narrative (and yes, there are changes in the verb tense during the scene), tells events from the past and connects different moments of the story.
If you'd like to criticise the way I understand flashbacks I encourage you to explain how you understand the term generally and its applicaitons.
I hope I've managed to do so without being utterly boring, as these things can get.
Nonetheless, and even if we don't consider that scene as a flashback I think that doesn't invalidate my main point: flashbacks aren't clunky in themselves, they've been used to much success in narrative before, they are as good as you can make them work, and simple exposition of past events is not inherently superior to them especially in a visual medium.
