The picture is for display purposes only, so don’t be concerned that it is going to be included in any message you send.
You can change the image in your account settings (Menu > Accounts) then simply click on the image.
You can choose from one of the included icons, or click on Browse to select something already on your computer.
The image that is inserted when you setup the account is taken from your Google settings, or from other sources like Gravatar where you may have previously registered an avatar.
Well, the issue here is that the specific profile picture em is using is not the Google account one and I haven never used gravatar. It is a random picture from my Google Photos collection. So I am very curious to understand how it was selected.