If you are a regular player on Steam, you may want to add some of your personality to your public profile. A good way to do this is to change your Steam profile picture with a custom avatar.
You can change the profile picture on Steam using the application on the desktop or through the company website. The steps for both methods are the same, so you should be able to follow these steps using either the client or the website.
To get started, open the Steam client on your Windows 10 PC or on Mac or visit the Steam website and sign in to your account. Once logged in, select your account username in the upper right corner and select "View Profile" from the drop-down menu.
In the menu for your account, click on "Edit Profile".
Select "Avatar" in your Steam account profile settings to change your profile picture.
In the "Avatar" menu, click the "Upload Your Avatar" button to upload a new profile picture. This image should be at least 184 pixels x 184 pixels in size.
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Alternatively, select one of the following profile pictures provided by Steam. To see the full range of Steam avatars, click the "See All" button.
You will be able to preview your new profile picture at the top of the menu once you upload it (or after setting one of the pre-existing options). When you are ready to apply the change to your profile, click the "Save" button.
Once saved, your new profile picture will be immediately visible to friends, followers and other users on your Steam profile (unless you decide to make your Steam profile private).
Source of information: howtogeek.com