SLIGHTLY MODIFIED VERSION!!
I'll try and explain how I do it, posting from an android phone...
On stats/results page, click "share".
Click "copy".
My phone comes up with a list of apps you can copy to. I click on Facebook "Messenger"
(FBM).
Select "chats"
I search for myself on FBM and click "send" on the right hand side of the screen.
Wait until the "send" button greys out/changes colour.
Close FBM/Triplit.
Open FBM and select yourself. (Next to your name it says "you sent an image").
Once you've selected yourself, you will see the image on your FBM feed. Tap on the image.
At the top of the screen, there is download symbol (an arrow pointing downwards touching a horizontal line).
Tap on the download symbol.
FBM tells you the image has been saved to your device.
Close FBM.
Open this thread on Mumsnet.
Type a random letter or some text (image won't upload without text).
Tap on the camera symbol, then the "mountain scenery" icon to access images saved to your device.
Tap on the image you want to upload.
On the Mumsnet tab on your phone, tap on this thread.
Tap on the space where you would type a new message. (Mine says "posting as Dapper").
You should see your text and image, all ready for posting.
Tap on the "post" arrow. You'll be warned that your image is awaiting moderation.
Tap "done".
That's the only way I've found to do it. If anyone has a quicker method I'd be delighted to copy it lol. And apologies if this sounds like teaching your grandmother to suck eggs!!