Add background image to imagecache_sample.png

Druplicon by itself is perhaps not the best image to use as the sample image, as its round shape can make it hard to judge where the edges really are. In addition, there are a few transparent rows and columns on each edge, meaning it's not really a true representation of what your scaled image will look like any way you look at it.

I propose this replacement, which adds a simple checkerboard background behind the Druplicon. In addition, by using pre-existing colors and OptiPNG, I was able to bring the filesize down by a few kilobytes.