I'd like to share my experience with editing these tab icons... This thread was excellent to provide the majority of the files I needed and the manilla editor to get me started... But I did have to get around a few hoops to get it all to work for me.
My solution was concentrated on changing several of the icons to match my iphone-ish theme, that has mtp tab installed using iphonetoday plugin and pocketdigitical clock hosted by the ultimateconfig plugin inside the mtp tab.
So these files were good to get a start for me, and while my images are not the cleanest/smoothest right now, I edited several of the icons using the manilla editor (on vista, w/ run as admin option used.) However, I had to locate the manilla files for the today flag tab icon installed by the mtp add-on. I opened that install-cab with winrar on the desktop and saw 4 manilla filename references. I located those in my phone's windows folder, and copied them to the desktop folder to use with the manilla editor as well. Then I had the option to change the today flag also.
Now this was all nice and all but then I couldnt get these files to copy back over to phone's windows directory, due to a permissions issue. I tried a few approaches, and maybe there's another way... but... I used MaxTTM theme changer. I havent really been able to figure out that program for the most part, so far, but I recognized one ability it had which was just what I needed here. It has a utlity section that allows backup and restore of manilla files... I copied the new manilla files that I made with the manilla editor & photoshop to a different folder on my phone (\tabs in my case,) launched MaxTTM (on the phone,) and restored from that path. It raises the neccessary security context and performs the copy of the specific manilla files to the windows directory on my behalf and then issued a restart to get everything loaded.
One last thing was that while the transparent slider graphic looked nice to me while sliding, it didnt look nice to me when it landed on something, because the icon on the slider was offset from the one underneath, and both would be seen, one overlapping the other... So I changed my slider to a solid gradient between black and a lightgrey...
There ya go... I got my changes made, and am happy with it all now... Have fun!