Hi tonightwetaco If you take a screenshot of the lock-screen then add it as a layer over the Wallpaper and reduce the layer opacity (transparency) you can see exactly were to crop.
If you don't have a GFX Program to do this Paint.Net (freeware) can do this (eg) >
Paint.NET - Free Software for Digital Photo Editing
It may or may not work for you for a number of reasons, but you can try.
First, make sure your screen is set to it's native (recommended) resolution, preferably at 100% dpi.
You'll notice that the sign-in screen is always dimmed, so you'll need to find software to take a full resolution screen shot of your Lock or Login screen(s),
'Ease of Access Replacer' is one, (Google it):
In your case, you're going to have to set your 'Dinosaur' wallpaper as Lock Screen offset so you can capture a 'dimmed' version of the Dino face without the user icon in the way;
in Paint, with Rulers, Gridlines, etc showing (1), select the area of the icon (2) taking note of the position and size (3) and (4),
then you would crop an image of the Dino face, in the same manner, of the exact size and apply it as your User Icon.
Then set your proper wallpaper as Lock Screen.
It may not land properly or be the proper shade but you can give it a shot.
Mine is a simple shade of gold so it worked: (if you look close though, you can see a faint outline cuz I took a snip of the dimmed image a little to the left of the icon)
thanks for all your effort Edwin