I don't, and I know canny lass doesn't (as she always checks her posts and edit's where necessary) so I can't think what is going wrong with yours. When you select the Edit option from the three options under the three dots in the top right hand corner of something you have just posted - complete your edit - then scroll down to the 'Save' button, bottom right of screen, and select that and the post + edit should be saved and no further action required.
I think the time you are allowed between posting a comment, and then the using the edit option is approx 10 mins. After that time the Edit option is no longer available.
I have now gone back to this comment, selected the Edit from the '...' - added the image - and the selected the Save button, bottom right.