So SketchUp 2017 Make? Please correct your profile to avoid confusion in the future. The steps for many things are different between SketchUp Make and SketchUp Free (Web).
In the desktop versions of SketchUp there are several ways to import an image to use as a texture. My preferred way follows.
Draw a rectangle to the dimensions of the area covered by the material. For example this one indicates 108 cm square.
Go to File>Import. Choose the correct file type and make sure Use as texture is selected.
Click Import or double click on the image. Then click and release at the lower left corner of the rectangle and drag across to the opposite edge and click.
Doing it this way sets the texture size right away and lets you see the texture as applied to the model. If you decide you don’t like it, one Undo step removes it. If you have more than one texture of the same size you can reuse the square as you repeat the import steps. If you want to compare several materials, copy the rectangle and apply each material to a different one.