Here's the proof anyway, modelled on my Alcatel IDOL 4 Pro:
The '+' control is uppermost in Winsta's hamburger menu. Note the Fluent Design transparencies, by the way; (right) You're then in an image gallery from photos on your Windows phone. There's this pop-up teasing further sharing features in the future.
So far the only image enhancement is cropping, but I'm sure some basic filters will arrive in due course; (right) add a caption and then 'Upload'. Job done!
Proof of the pudding - the photo now on my Instagram timeline; (right) sliding over the hamburger menu, again showing transparency, with the lights 'glowing' through. All very well done.
Browsing my Instagram media in Winsta's view of my profile. I don't use the service in real life, which is why there are only 13 test posts - yet I've accrued 108 followers - somehow!
Winsta UWP is highly recommended. There's a lot to like about Winsta's speed and UI, it's a breath of fresh air - you can directly encourage the developer by buying Winsta, since it's a couple of quid in the Microsoft Store. Go on, if you want to see this updated further, buy the developer a cup of coffee in this way...