After some testing, it seems that images from my DSLR are fine (4700 wide) but images from others are not, it seems 5000 pixels is the cut off and bigger than this produces the error.
I set my forum to resize to 1000x1000 for posts, if I remove that limit, images >5000 pixels upload then OK but are not resized and are treated as attachments not thumbnails.
So it seems the act of resizing the image to 1000 pixels, if the source image is > 5000 pixels to start with, causes the error.
I spoke with my web host to see if this is a hard limit on my hosting package and the response was:
"Image too big" sounds like a setting in the script to me. I don't think GD gives text output.
Max attachment size is set to a huge 100MB (100024KB).
Alternatively is this indeed coded into the forum and if so, where can I change it please.
The reason why this is important to fix is that so many cameras these days are churning out big photos and so many of my members don't have the technical skills to edit and resize them before uploading, so the forum needs to hand hold them.