An avatar is a render, not a photo
Roblox avatars are different from every other platform on this site. There is no uploaded photograph. The character is a 3D model, and Roblox renders it to a flat image whenever one is needed. That render is what you download here.
Because it is generated rather than stored, the image updates whenever the character changes. Download it again after changing an outfit and you get the new look.
How the lookup works
Roblox's thumbnail service works with numeric user IDs, not usernames. So a username lookup takes two steps: the name is resolved to its ID first, then the render is requested for that ID. If you already know the ID — the number in a profile URL like roblox.com/users/1/profile — you can enter it directly and skip a step.
Size, measured
The thumbnail service accepts a size parameter, and the values it accepts are fixed rather than arbitrary. Requesting 720×720 returned a 92 KB PNG in testing, against 39 KB at the common 420×420 default — well over double the data. This tool requests 720.
"Avatar is still rendering"
Occasionally a lookup reports that the render is not ready. This is normal and temporary: it happens when a player has just changed their character and Roblox has not finished producing the new image. Waiting a few seconds and trying again resolves it.
A note for younger players
Nothing here asks for a password, a login or Robux, and no legitimate tool ever will. Any site that asks for your Roblox password to show you an avatar is trying to steal the account. Avatar images are public and need no credentials at all.