I had this problem  also, but figured the solution. I am not sure
why, but when I put  the new logo  file in the same folder, I
neglected to check the NTFS permissions, and I had not altered one
of the original pictures (therefore it did not have the same
permissions).

Needless to say, I got the exact same problem. If you compare the
permissions on your new file  with others, I believe you will find
they do not match. If you change  the permissions to match, it should
work fine.

I did not enable anonymous, though it seems that might work. My
customer is pretty picky about security.