I got this as the response for some of the images (quite a few):

After opening them in vim the contents of the 12 bytes seems to be Unauthorized.

I'm assuming it was because of how fast I was trying to get them all at once. I originally wrote this with a different goroutine for each image to do all at once, but I got "too many open connections." Maybe I should add a little bit of a delay before each image downloads.