You need a mod like Portraiture or one using PyTK's scaling feature to use higher resolution images. Both of these mods require SMAPI, which is an important first step to getting any mods to work! Overwriting game files directly is more of a pain to undo, easy to do incorrectly, and most of the mods you find that rely on this method will be woefully out of date compared to the game version.
If you're on Windows/Mac/Linux/Android, we can help you install SMAPI so you can install Mods by unzipping them into a mods folder, without having to overwrite :)