I suppose that it is the ThingWorx Platform login web page modified.
We are in the ThingWorx Studio forum.
ThingWorx Platform and ThingWorx Studio are not the same software but I will try to help you.
So, I think that's you have modified the FromLogin.jsp in <Tomcat install folder>/webapps/Thingworx/login/ folder.
Can you confirm this point, please ?
If it is correct, I suppose that you can add in <body> tag a background image property as described in this article :
Another way to do that, is to use stylesheet as described in this webpage :
In the same FromLogin.jsp, in <style> tag, in body style, replace background: <%=backgroundColor1%>; with the syntax to have an image will be enough.
By the way, FromLogin.jsp is a standard file of ThingWorx Platform.
So, I am sure if it is supported to modify it.
But I am sure that when you will upgrade to a new version of ThingWorx Platform, (deploy a new war), you will need to apply your change again.