In the area where you edit your text, there should be a little square image of a tree. Click it, and it will prompt you to upload an image from a URL. So you will have to have a direct link to your image to post in the space given. Once you do that, and hit "OK", your image should appear just fine on your post!
If you were referring to an avatar, or a profile photo. Go to the top right-hand corner of the page ,and there are your account settings. You have a variety of options to customize your profile from there.