If in doubt--Use the HELP section on DK. There are many good tutorials on how to use this site :)
Go to your page and just underneath your current photo--select photo.
Upload a new photo of your choice
Then--SELECT SAVE!
The most common mistake is not SAVING your changes. You will find the SAVE option all the way to the end of your page on the bottom right. You will have to scroll down to find save.
You know, from time to time I change mine and sometimes it won't let me, It is usually because of a "glitch" but not with you. I sometimes reboot my computer or wait for another day and voila....it works! Go figure! Just make sure you have your photo saved in a small resolution and as a jpeg. If a high resolution (over 100 dpi) it will be too big a file and might not load. Though I understand you are not even at the "load" stage. Every now and then, when I mess with my page nothing will work and even the background and side pictures won't go. Just a thing called #@%&* computers!!
Be sure to click save when you change it. That is the mistake most of us make. From reading the posts, you aren't able to get the picture to load and the save advice is moot. However, it is still what most of us forget.