I figured it out, thanks Bettina

There is no option in the media library to set images to full size. Maybe I am missing something. Would you be able to tell me how I can upload them in full-size. The actual size of the images doesn’t seem to make a difference. There are several that have the same dimensions, some of them appear blurry, others don’t. Also, the images in the mega menu are smaller than in the article, so why would they appear blurry? By the way, I have the same problem when using the Ad Block.