Lol. I did the same thing!

Dear Guardian, I am assuming you mean your Bungie.net profile image. It is a rather simple thing to do but if you're unfamiliar with the site it can be hard to find. Click on the image of your profile in the upper right hand corner of your browser and select "Settings". You should be instantly on the page that has the name, alias, motto, avatar, and background settings. From there, go under the avatar images and select one and once you have decided, click the save option on at the bottom of that section. You should be able to refresh the page after a moment and see the changes you have made. I hope this was able to help!