Really simple;
Click under the pic you want where it says IMGcode. a tiny yellow box saying 'copied' will flash on.
Then to Xmarks; in the 'Repy' box, right click and select 'paste' ; A row of code stuff appears (that will be your pic when you submit your reply). It took me days to work it out, but Mc murdo & the rest of the rabble kept at me until I finally got it right.