click on your name in the upper right of the screen, this will take you to your profile page under the default pic there will be text link that says "change", click this you can then choose to use one of the images ASK provides or an image of your own (please be sure it is sized down to aprox the size of the images you see displayed or it may not load) Also you may need to do this from the website and not a mobile app as some features are not avail on the apps.
Click on your profile (upper right corner) and click on "change" under the profile pic. Then you can browse your images you saved on your computer/laptop and then upload it by clicking "upload". It might take a while! It is easy, that is how I put my profile picture as a fairy Pikachu!!