Try this:
Have the image on your phone or device (forget linking to external images or trying to embed them/codes etc).
At the bottom of the composing a reply/post is 'upload a file' (or possibly a 'more options' button - press that if upload isn't visible to lead you to the 'upload' button).
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When that is clicked, you will get the usual PC/MAC file/folder pop-up to locate any image you want to share.
On android, this pops up, for example:
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When you've selected the image you want, it will upload to the site and appear below the reply/text box that you are creating and give you 2 choices:
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Insert a thumbnail or full image.
Wherever you have the cursor in the text box, the thumbnail or full image will appear.
If you click neither, it will appear as an attachment at the bottom of your post (which doesn't look great and can be fixed by editing your post and re-inserting the image in to your reply).
Try replying to this with an image inserted as shown above.
If it isn't working - PM me for more help.
Regards,
FWD.
Example of an image that isn't inserted as thumbnail or full image: