Unfortunately updating the logo in Gmail is a bit more involved. I posted about it here. If you use this method, you're going to want to update your Gravatar image. However, if the word "Gravatar" scares you, there is another way.
The process is to use the same image two times, one for your Gmail profile picture and also for your email signature. You can't just drag the image into Gmail like you can in Apple Mail.
First, we're going to change your profile picture in Gmail.
First, we're going to change your profile picture in Gmail.