I think the caption on the picture is wrong here for 24" iMacs - it says: "On the 24" iMac Intel Model A1225, the bottom center two screws are long (26mm), four on sides (two left, two right) are medium (18mm), and the remaining 6 (four top, two bottom corners) are short (14mm)." I think actually bottom centre two are long, then all the rest are medium apart from the two either side of the CD/DVD drive
I found it was not necessary to do this step if you do step 20 first - i.e disconnect the temperature sensor cable from the hard drive, but not the logic board
You don't need to do this step at all - you can leave the panel connected to the circuit board while you replace the hard drive. I did this while replacing the drive on my 24" iMac, just resting the panel on a box I had put next to the iMac on my desk, so that the panel stayed at the same level but was just moved and rotated sideways, with the cable still connected.