Provide "Powered by Drupal"-Icons with transparent background

It can be desirable to have the "Powered by Drupal" icon in the footer be surrounded by transparent pixels, rather than blue, black or white ones.

Reproduce:
Change the background-color of your theme to something not matching the "Powered by" block configuration.
Look at the image in the block. You'll see a frame of 1 pixel width in an unmatched color around the Drupal logo.
- or -
Working with different themes with different background colors, at least one might not match, but the block has only one single configuration.

Solution:
I suggest adding a new set of icons like the attached ones that will blend in almost any background, nicely.
Of course, this set needs to be listed in the block configuration, see the attached code patch.

Further work:
The attached .png images were drawn and compressed with GIMP, so further optimization might be desirable. (See #552000: Smush core images.)