It seems likely that the color space in SketchUp is different from that in your image. That is, in each situation the binary R,G,B values in the image are translated to screen colors via a lookup table. If a different table is used, the hues may shift. Photoshop, for example, has an option to export an image using colors suitable for web pages. An image downloaded may already have a color space that is compatible with SketchUp whereas your image may have a different one.