Further to this problem, and to assist Juan, I have been on the SMF forum support and they have come back with the following possible fixes for this problem:
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you issue is -- with fair certainty -- the issue noted in the first response.
Your system is choking on memory... if the mod didn't help, then I suggest checking with your host.
Yes the memory was indeed the problem in my forums...no idea how but it was trying to allocate waaaay too much ram and after I adjusted it from its enormous amount down to 1024 for each forum the members can attach images to their posts again.
try installing
https://custom.simplemachines.org/mods/index.php?mod=4111 and see if it helps.
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I have had to start a new thread as experimenting with the original thread and attaching various images has blown it up.....sorry!!