Hahahhah!
Sorted the issue...

It was not the file size, or the code.
In the names for the pre-set sub-folders I had spaces.
That causes any image upload to fail.

Your instructions do not seem to specify that limitation?

Steve

Surely, you can not use spaces in folder names. Use underscores or dashes to separate words.

Spaces are not universal, take too much care of their URLs, which should replace a space to %20, also on many servers copying to a folder with a space is a big pain.

I'll take a look in README later and will add this note. Thanks.

I've checked the current README for this add-on, and it strictly says:

NOTE: Try to avoid using apostrophes or any kind of non-alphabetical or non-numerical symbols in folder names! The script may work improperly.

Anyway, I will add a few notes about strictly spaces as well.

Sorry — yes, I read that but just didn't think about spaces as well... sorry!

Steve