@jonatan-moonchild Hi! Worry not, you are not disqualified.
Edit your (first) post and add the bigger size image at the bottom (with a short explanation if you want). Leave the original image there so that there's doubt about your submission being on time.
To stop the forums from resizing it, you need to make the file size smaller - either make the image a bit smaller, or compress it a bit.
@flamer you can leave it like it is.