Hi David, I have allowed images to the post from other pages, and also you can attach images to the post as a attachment. Images can be up to 900 x 900 px and the attached file (from yuor PC) size must be less than 1 MB. I think it is enought for most cases.
I have tried to attach few images from Your site ant got the same error. I checked all setings in the forum system, and also some hosting server setting, everything seems to be OK. Then i tried to attach image from somewhere else and success this is from hackaday page,
From link I see that hackaday.com also use wordpress. So maybe there is some prolem with your page or maybe with the images, it coud be that images are somehow in wrong format, and forum script can not determine the image size from your site... This is just my suggestion