I'm trying to change my login screen to have the same background image as the lock screen (Pic related is an example of what I want) However I am only able to change it to a default color instead of an image. Do I need to perhaps register something within the registry in order to change it, or do I need to install third party software? Thanks for the help.
Did you even read the post? As far as I know It's not possible to change the background of logon screen in Windows 8.1 to anything other than one of predefined colors. Wallpaper is possible only in Windows 7 and Windows 10 (1607 and later).
I installed 8.1 a few days ago too. (Was previously using LTSB 2016). I was trying to find even a hacky method to do it. No luck thus far.
But why do you need it there? Look at this picture as much as you want after logging in. Logging in takes only about one second or less why is the picture needed for so short time?
Most likely because many consider a different logon screen ugly/disruptive and want to keep the same transitional flow from logon/autologon to desktop. Win7 lets you do this nicely.