After investigating I have found its just an issue with how smf handles image uploads, mixed with rather large images.
One way to deal with this is to break the post into multiple smaller posts.
Alternatively, and preferably
upload the image files to the gallery linked in the top right of the forum, then include it in the post with the *IMAGE* url using the tags below.
To get the image link, once uploaded in the gallery right click the image and select "Copy image URL" or the similar depending on your browser.
Or using the tag icons above the editor box
If correct it works as such
Other sites on this same machine using other software packages employing php are dealing with comparatively much larger files and all of them are working well. SMF just does not have a great reputation of handling images well, and its showing here.
I have checked, php is set for VERY high limits, but for some reason the smf software is just not, or not capable of doing so.
I am sure with enough effort I could make it work a bit better, in fact at one point there was a project that merged smf and gallery2, its no longer maintained, and hard(ish) to do with a established forum. So unless the need really comes about, I hope this workaround is acceptable.