You may need to resize them.
Many of the older ones were 640x480, leftover from the old
dial up days, which appear disappointingly small nowadays.
Try again, via right-click on an opened online picture to
copy image address then using the "insert image" icon in the forum's toolbar and pasting the url and hitting
insert...
(interestingly, the pictures ALL showed up in the post on my screen, but were represented as an X in a red box when previewed and/or posted)