Artifact in Bicubic interpolation
What you're seeing is acutance, as described in the wikipedia article on bicubic interpolation. 

However, for downsampling an image, cubic interpolation is perhaps not the ideal choice, but makes more sense for upsampling.

Note that if you're downsampling by more than a facor of 4, there will be many pixels in the original image that are simply never used in the downsampled version.

The easiest way to get around this is to downsample by iteratively taking the mean value of 4 neighbouring pixels, until you're within a factor of 2 of the desired image size. Then you can do the last part by cubic interpolation.

If you're want high quality resampling, try taking a look at the ImageMagik documentation for inspiration. In particular they recommend using windowed filters for downsampling, such as Lanczos or Sinc.