When I post an image I usually resize to that it has a maximum dimension of 500 pixels, which fets a file of about 300 Kb.
If you just post the image - as I did with the one of the Christmas tree in the window it should, I think, show as a thumbnail which can be clicked to enlarge.
If you upload an image and click on 'insert attachment' then it will appear in the message (at the point where you curser was when you did that) at the bigger size.
[ You are not allowed to view attachments ]
This is what I do using a laptop; not sure what the differences might be if you are using a phone or a tablet.