Pete its not the width thats the problem(all the images I tried to post were 1000 pixels wide), its that the image checker freaks out randomly on some picture or pictures and instead of posting it, just brings you to blank page, then rather than go through each image to find the one that it freaks out about(because it will not freak out about that one image if it is posted by its self).
So if you have 5 images, by trial and error you discover image #3 causes the program to freak out, you remove it, then you can post, then you can post the image by itself in another post.
all in all though its just really really frustrating.
If we want to keep the image sizes down I would suggest ditching the image size checker and go with a reducer, many forums have this in place so that an image over the size of whatever gets scale down. LOST is this way and it seems to work nicely.