This looks great, Xenophage. Thanks for creating it.
I have reached out to Meggan, who initiated the main petition on change.org (http://tinyurl.com/saveCOH
), and offered to resize your image for her so that it would fit on that site (she currently does not have a picture).
I have not heard back from her yet, but I did a quick test in PS. The image size on change.org seems to be 556x304 px, which is a bit different than your facebook banner. I am usually very much against messing with aspect ratios, but the result looks very acceptable (I am sure you can check it yourself very easily). I think this is fine, but I just wanted to run it by you.
I'll let you know if I hear back from her and she's going to put it up.