The pixels for function keys are stored in 

/u/ccr.17/pixmap


You need to know the screen resolution first
easiest way, run sieat, at prompt, when you exit it will say

2048x1152.qss

change directory to this resolution

/u/ccr.17/pixmap/2048x1152.qss

under these directories there is

tk_f1_on.png   
dk_f1_on.png
ek_f1_on.png

tk - transaction key
dk - default key
ek - extension key  (row 2 sieat - eg. alter orders)

F1 - function keys - there will be one for each function key

you need to edit these files so you can look at what pixel they have


gthumb tk_f1_on.png   should say something like 130x95 


++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

to see the resolution you can also do this:


export SHOW_GUI_DETAILS=1


then run sieat and mouse over the function key
this will show /u/ccr.17/pixmap/2048x1152.qss