Why am I getting a blue box with question mark instead of logo image? It happens in my signature (in gmail) and in mailchimp (again, only with the logo).

The question mark in the box, if it looks like this: means that safari cannot find the linked image. On windows machines, this would be the equivilant of the small "page-with-a-red-X" logo that appears in the same scenario. Check the reference to the image and make sure that the image is in the correct location, and is accessable by the reference.