In the past I have noticed that the full name of the account holder was sometimes displayed too, before entering the password. But not systematically, it seemed to be account-dependent. Perhaps the OP can confirm this.
There is a potential breach of privacy here, even if you put a picture of your cat. Because many people reuse the same image for their profiles on other sites such as forums or social network sites. A simple reverse image search can often match a picture with other profiles and ultimately a real identity.
This is one of the most basic doxxing techniques.
If your real name is already embedded in your Gmail address, you probably don't care. If you have some made-up address, that you use to register, post (and sometimes troll) on forums, maybe that should concern you because you are not as anonymous as you think you are.
Think of it, at one point, we have all upset someone with something that we wrote, for good or bad reasons. As a result, we may have had to deal with stalkers at least on one occasion.
Everything you post can and will be used against you. A picture is very often uniquely identifiable data and the metadata sometimes tells a lot: for instance a raw picture from a tablet, camera or smartphone will often have GPS coordinates embedded in the EXIF data. Plus the name and model of your device. Normally the thumbnails will drop all EXIT data but you still have to think twice before posting or uploading anything.
The EXIF can even get you in trouble with the tax authorities. A famous example is late french singer Johnny Hallyday betrayed by pictures posted on his Instagram and Twitter accounts. The geolocation metadata revealed that he spent little time in Switzerland, where he claimed to have fiscal domicile.
Therefore, I recommend using exiftool to systematically strip all EXIF data from your pictures before uploading them to a site. As easy as:
exiftool -all= picture.jpg
The tool has plenty of other options, including bulk options. You could also fake some of the data if you want to have fun with adversaries.
