Hi,
First of, welcome to AC
I would suggest you upload a photo using a PC / laptop. When you use a tablet or phone it will only let you upload a certain file size.
Click the below link to this fortnights photo competition:
http://forums.androidcentral.com/photo-contests/566087-android-central-photo-contest-clouds.html
Down the bottom left hand corner of every page you will see a button called "Reply" press that.
this is where you now upload your photo, hover over the icons to see which one is for Images, select it and upload it
Good luck.