Textures should be in the same folder that the .3ds resides in, which is usually under objects\[project folder] where project folder is the folder you designate for a particular ride. If you're just messing around and aren't using an object for a project, then just make a folder for miscellaneous use. (Unless you want it to get buried under masses of random .3ds files and textures from old downloaded rides and from rides made by users who can't keep their files organized at all.)