A mupload is simply a "mobile upload" that you can put on the internet, most commonly Facebook or twitter. All cell phones that have access to the internet can make a mupload. Once you snap the picture on your cell phone, you are immediately given the option to upload your pic to the internet. Instead of saying "I just put a mobile upload pic of you on facebook" it can simply be stated, "I just muploaded a pic of you one facebook."
When my roommate fell out of her bedloft, I immediately whipped out my cell phone and took and picture and sent my mupload to Facebook.