Well, you'd obviously have to add some height as well. I was focusing on the horizontal aspect of the resizing to hopefully illustrate my point about creating a space for your desktop icons so they don't block the characters. Adding height should be simple enough, I would think. Once you get above their heads, it's mostly just a solid field of grey (I think it's grey, I'm colourblind though).
Another idea would be to incorporate the storefront (as seen in the first frame of the strip) into the picture to create that extra space for the icons. Like I've said before, I'm not experienced with Photoshop or related programs, but it seems to me that after cutting Marten out of the picture, fixing the storefront should be simple as it's all straight lines and solid colours (except for the lighting effect on the windows).
Anyways, them's my $0.02.