You can customize your toolpallets quite a bit. You can add custom images in place of your pictures. If you take a look at the attached images you can get a good idea of how to change them.
Otherwise I belive that the 'thumbnail' defaults to what ever your 'drawing preview' is when you open or save drawings. If you are zoomed out, that is what your thumbnail will show. If you try zoom extents, it may make the pictures a little bigger.