Any image your browser can show can be a texture: PNG, JPEG, WebP, GIF and more.
Open images along with the model, or later, or drop them onto the views: each goes to the material that names its file. Or drop one onto the texture box at the right of a material's row, in the Textures section. See Materials and textures.
A texture covers the 0-1 square, stretched to fill it whatever its shape: a 256 by 64 image lies over the whole square just as a square one does, as every renderer lays one on. UVs outside the square repeat it.
A PNG with clear parts shows them clear on the model, and so does an opacity map for its material. In an .i3d model, a material with "!" in its name keys out a color of its texture.