The default flower profile picture is actually a BITMAP file which is stored in following folder:
%ProgramData%\Microsoft\User Account Pictures\
Copy the above string and paste in RUN or start menu search box and press Enter. It’ll directly open the folder in Explorer window.
The BITMAP file name is user.bmp and its a 128×128, 24-bit bitmap file.
We just need to create a new BMP file in MS Paint, Adobe Photoshop, GIMP, etc but we’ll keep its size 128×128 pixels similar to default BITMAP file.
Once you finish creating the desired BITMAP file, save it at any location with user.bmp file name.
Now you’ll need to replace existing user.bmp file present in “User Account Pictures” folder mentioned in STEP 1 with your newly created BMP file.
Before replacing default user.bmp file, you should create a backup of the file so that you can restore it later if you want. To take a backup, you can simply rename it to any other name likeuser_backup.bmp.
Now copy new user.bmp file to “User Account Pictures” folder.That’s it. Since you have replaced default BITMAP file with your new one, now all new and existing user accounts which have their profile picture set to default flower picture will automatically start showing new picture which you created in STEP 2.