i miss old league and i want to change some of the old splash arts back.
etc
can i just manually swap out the splash arts for the art i want to be used in game? or will this get me banned? i know riot doesn't like modifications to the client, but it's only cosmetic changes so im not sure.
No, but 3rd party apps and changing game files are "use at your own risk". You may inadvertently flag your account for abuse by doing so, and making changes can also corrupt your game files.
Most likely, it's really not worth the risk for changing a picture you see for 1 minute...
does that even work? as far as i know the game doesnt open when you change certain things.
you should ask the riot support
