HELLO ALL, I just signed up, to tell you I found the solution....
Christina here,
I am so so so happy, finding your forum was the very FIRST time I have even heard Anyone with an even similar issue to mine,, despite trying to look for this answer over and over, BUT, because I was looking for how to upload my own pictures into Marketplace ad listings, all it would wind up giving me was posts on HOW to sell on Marketplace, not how to access my ACTUAL camera storage & not some RANDOM Mish mash of photos....
UNTIL NOW....
TRYING to break into selling on Marketplace from years on eBay, if I needed to access previously taken photos this was a PITB... (I don't swear, but I think you get my drift) my issue was always with items I had already pictured and had saved in my phone or my Google photos account, yet I could NEVER access them on the Facebook app....
TILL TODAY!!
YAY

I was so happy to find that even one other person had mentioned something CLOSE to my issue.... SO, I read ALL of the replies and started getting discouraged, AS USUAL, NO FIX or solution...
But I kept reading, in hopes!
And, THEN, read CHANCHAN05, (major props, man)! And, he suggested maybe just trying out in the browser... & although he didn't offer it as an absolute, diffinitive solution I decided to try this!
Because, knowing how I do the difference of using full-sized, desktop versions of web pages on any Android, esp my S8+, lol, although a bit difficult to navigate, oftentimes gives me so MUCH MORE capabalities instead of using mobile adapted pages, desktop ALWAYS increases the usability of web pages than those that get adapted for mobile....
And, since I was EXTREMELY hopeful, given my knowledge, I JUST HAD NOT THOUGHT OF IT UNTIL IT WAS MENTIONED...
Guess what, ChanChan.....?
Cha-ching, you da man (or woMAN) LOL
SO, I am so ecstatic add all get out, because it 100% MOST DEFINITELY works...
I'm ecstatic now I can sell on Marketplace on Facebook without having to go through multiple multiple steps to get pictures into that gallery on Facebook and I would do all kinds of stuff I would upload & share them privately to MYSELF on my Facebook app, & on and on to no avail and none of it it would work anyway....
So needless to say I'm extremely ecstatic thank you thank you thank you and if you can't tell I am one extremely happy happy camper so thank you Android Central and I WILL be back, because
(My new signature, tag line, gonna copyright this, FOR REAL)
I am an Android FAN....
"Don't EVEN TRY to show me an, Apple, a Mac, we won't have none of THAT...
ANDROID FOR LIFE, baby!"