I have the same phone as you and its a pain to explain..
But i will try.
Ok i asume you access this site via your phone and the photos you want to upload are in your phones gallery.
So you make the post as you have done above,
Before you click the "submit reply" button
Scroll down the page and you will see Additional Options
The first box is Miscellaneous Options (you don't want this one)
Second box is "Attach Files" this is the one you want so click "manage attachments."
a popup page will appear,
top left click "chose file"
Another pop up willappear you need to click My Files
Now you have 2 options, internal storage or sd card, i will asume you photos are in sd card (mine are there) so click sd card
Now click the folder DCIM then Camera and your photos will appear on your screen.
You can only upload 1 file at a time so on the left column click the photo you want to upload
Got to top right hand and click Done
on the next page click upload
Now if you get a failed upload message the your file/photo is to big (megabytes)
you will need to resize then down to something under 500kbs
I use Photo &Picture Resizer from the play store its free and works great.
If successful upload then you just repeat for any other photos.
once you have uploaded then click close window bottom middle of the page
then click submit and your done.
I am sorry but i can't explain it any better than than that.
Good luck and congrats on your new lady friend.
This is the one i have just uploaded via my s7 edge
Samurai..