I just build a 1.3-SNAPSHOT. There are still two problems present, both due to the img.poweredBy being inline rather than block-level content:
- If the images are small, several may end up next to each other.
- If an image is larger than the sidebar, however, its edges are cut off.
You can experiment with this by explicitly setting the logo's width in your site.xml.
If you move to block-level content, you avoid the first issue as non-floating blocks cannot sit side-by-side and the second issue because the contained block pushes the boundaries of its container (the sidebar).