That's odd Toby.
I deliberately tried to load a very large image just to test and I copied the image and pasted directly in the text box, which works with a smaller image.That's odd Toby.
Are you clicking the Attach Files box or going in from the Toolbar.
Perhaps you have an iPad and try to attach the same pic?
I am no techie, certainly not enough skills to offer advice.
Everything has limits.I was under the impression the forum software resizes the pictures to a smaller size and lower quality automatically, hence why some people choose to post via photo hosting page links instead.