I've never uploaded images from my phone for viewing on here but, in response to requests above, I've just been playing around to see how to do it. Here's how it works on my phone:
In your web browser, go to postimages.org
(
https://postimages.org )
Tap 'Choose image'
If you're asked for permission for your web browser to access your images, choose either 'While using this app' or 'Only this time'.
You'll then be offered some file locations. On my phone, I saw both 'My folders' and 'Folders'. I guessed (correctly, as it turned out) that 'My folders' was the one I needed to use. So I tapped on that.
I was then offered 'Camera', which I again tapped on.
That displayed all of the images stored in that folder. I tapped on the one that I wanted to upload to place a tick next to it.
Then I tapped on 'Done', which resulted in the file being uploaded to Postimages.org
The next step is to long press on either of the top two links and then select 'Copy'.
From there, come to an AB post and (where you want the link to appear) long press and select 'Paste'. (It's always best to give any link a line all to itself). Then submit your post as normal.