Technically you could make custom graphics, and even music for our games. The sheets and music are all external. Now, I don't intend to SUPPORT custom graphics or music (like having a section on the site that will probably be made down the line, for example), but people would be free to do what they wish to the graphics and music files.
The collision masks and such are stored internally, so if people who do edit the graphics files do something ridiculous, like make a knife super large to take up the whole allotted space in the bounding box given to it in the sprite sheet, it wouldn't make a difference to where collision actually happens. Those who do silly things like that will only hurt theirself in that regard, since the visuals wouldn't match up with the collisions.
The collision masks and such are stored internally, so if people who do edit the graphics files do something ridiculous, like make a knife super large to take up the whole allotted space in the bounding box given to it in the sprite sheet, it wouldn't make a difference to where collision actually happens. Those who do silly things like that will only hurt theirself in that regard, since the visuals wouldn't match up with the collisions.