I mentioned it in another post.
If the assets are coming with the game engine internally and are not part of the source files you share with the project, you can use those assets. Some version of the RPG Maker use internal assets like that: The actual images and sound files are stored in the install folder of the engine and loaded from that location. Hence, nobody can access those assets without having the game engine and, also, those can't be used in another engine due to their limited license.
If the version of RPG Maker you're using includes those images or sound files IN the project folder (a.k.a. source files), then you can't use those as those are actually restricted from being freely shared online. Redistribution of the RTP assets is restricted with every version of RPG Maker that gives free access to the included images and sound files.
In other words, if you can open the source project file in RPG Maker just fine "as-is" without those RTP assets on a separate PC, it's ok.
If you try to open that project on another PC and it tells you that some files are missing, then you're out of luck for those RTP assets as they can't be openly distributed like that.