OverviewWhat are assets?
Anything that is used as a resource can be called an asset. Graphics, music, sounds, scripts, art, writing, or anything else along those lines can be used as an asset for one's project.
An asset is owned by its author upon completion. When one creates a completed tangible work then the author has the copyright automatically. It is up to the author who may use their work and how it may be used. Copyright law has some variation internationally as there is no international copyright law to be applied. It is common for all rights to be reserved which means that the owner of the copyrighted work reserves all the rights granted under copyright law.
Permissions and Licensing
One should always ask the copyright holder for permission to use their work in a derivative work. When permission is granted you are given license to use the work on the terms of the license that has been agreed to. Usage of a copyrighted work without permission or outside of the terms of a license is called copyright infringement.
Public domain, Creative Commons, and Copyleft
A copyleft license uses copyright law to enforce a license that allows freedom of distribution for copies and modifications of a work. Public domain contains all of the works that are not under copyright. These are two distinct areas that a work may fall under. Creative commons allows for a copyright license to be public and may have attribution (give credit), share alike (copyleft), non-commercial, or no derivative (no alterations) stipulations that should be adhered to. This gives the public permission to use a work - and only permission within the scope of its license alone - for their own use.
Free software is any software that lets you (1) run a program as you wish, (2) see the source code and make modifications to study the program, (3) redistribute copies, (4) redistribute modified copies of the program. This is not to be confused with freeware which is any program that is free as in price. SMBX was originally freeware, but could not exist as it is today without free software. PGE, LunaLua, and the release of SMBX's source code under the MIT license made the game free software by respecting the FSF's four essential freedoms that define free software. Free software powers the SMBX website and the forums too! Using free software means that one is free to use software as they wish without outside restrictions. Proprietary software, or when an essential freedom is missing, imposes limitations on users. Software choice should be carefully considered given the choice between proprietary and free software. The FSF has further information about free software licenses here: https://www.gnu.org/licenses/licenses.html
Every original work one makes is protected by copyright law and this applies to assets made for SMBX. Anyone can reserve their rights, or choose a license agreement for their SMBX content. Releasing a derivative work under a license that contradicts the copyright of the original work is not protected and constitutes copyright infringement. If no license is available it is to be treated as all rights reserved. Every tangible thing that can be used in SMBX is covered by these concepts.
Assets can be obtained from several sources. The LunaLua, and Graphics forums contain scripts and graphics respectively for use in SMBX. Permission should still be requested to use assets in one's levels and projects. Outside of the SMBX community there is no shortage of content to be found. Spriter's Resource contains sprites that can be utilized - as long as permission is granted and followed - for customized graphics. Music can be found in a variety of sites, and in the public domain or with creative commons licenses too! The Newgrounds audio portal contains all sorts of content with licensing terms that are readily available! Assets can be downloaded, and enter a project resource pool to benefit one's creativity and also respecting other creators' work.
Free Music Archive
Open Game Art
Free Software license commentary