Help with Cover

I'm getting a little frustrated because I can't seem to post my cover. It is a jpg file, it is 400x600 pixels, but every time I try to post it, it tells me it has to be 400x600, which it is.
What I do get is a kind of double image, almost side by side, so it looks like: Cover... Checkerboard pattern... Cover... Checkerboard pattern.
I don't know how to post that image to this thread.

So what am I doing wrong?

Re: Help with Cover

If you have a website, you can post the image there and link to it from here, so people can see it, download it, and look through the metadata to see if anything looks out of whack. If not, you'll have to post it to imgur or some other image host, but I'm not sure if they allow people to download the original file, or if they compress it or something. Might still be helpful, though.

EDIT: In case it wasn't clear, this is so people can look over the image itself, and its metadata, to see if they can figure out why you can't use it as a cover. Your post mentioned you didn't know how to upload it to the forum for us to look at.