Increase image size.

The test passes witrh an increased image size. As such this test isn't
concerned with the image size and a small image is used for convenience
only. However, if the image size is small than 3x3 then the method
call to getThumbnailByLongestSideSet() in library.createImage() fails
to return a thumbnail ID. This may be a bug in the ThumbnailService or
a limitation.