In order for you to set a profile picture, your Word Crack account needs to be associated with a Facebook profile. This way, since Facebook is in charge of supervising that the photos comply with the rules, we avoid the possibility of users using inappropriate images.
You can find more information on how to link your account to a Facebook profile in the following article: How can I link my account to Facebook?
If you do not associate your account with a Facebook profile, you will be assigned a random and you won't be able to change it.