Out of curiosity I put FW 2.6.1b on my Touch (old model). Png images show in epub (16 grey scale with distortion). I did not notice any other changes. There might be some, but I didn't know what to look for. The huge blank space at the top is still there.
After installation, the buttons didn't have a frame and the captions were very small; check boxes were also very small. After power cycling it returned to normal. I saw this under FW 2.5.2 too.
This is how to upgrade an old Kobo Touch from 2.6.1 to 2.6.1b:
Spoiler:
Prepare a KoboRoot.tgz containing these files (with paths) from FW 2.6.1b Mark 4 and copy it to .kobo.
Code:
usr\local\Kobo\libnickel.so.1.0.0
usr\local\Kobo\nickel
usr\local\Kobo\plugins\libaccessplugin.so
usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\libQtGui.so.4.6.2
usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\libQtNetwork.so.4.6.2
usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\libQtSvg.so.4.6.2
usr\local\Trolltech\QtEmbedded-4.6.2-arm\lib\libQtWebKit.so.4.6.2
Warning: It is important that FW 2.6.1 is already installed on the Touch, before you upgrade to 2.6.1b. Alternatively, you can make the respective changes to the FW 2.6.1 Mark 3 file itself.