I figured it out. Instead of sending a "CLIENT-ID"
header, you need to send a "BEARER"
header, with the token that you recieve by using imgur's oauth 2 api. This allows you to post images to a user's account, whereas before I only providewd the client id, which allowed me to post anonymous images. The code looks like this:
client.Headers.Add("Authorization", "BEARER " + accessToken);