The number of the .png should be between 000 and 324. Some numbers are available, others will replace an existing Logic icon.
You have to experiment at bit or open the Logic app resource folder and have a look yourself in order to find out which numbers are available. I can't remember right now.
Try something like 210 and up.
If your icons look blurred when they are zoomed in, you either used a very low res picture to begin with or you did not follow my guide precisely.
Currently working on Vinnie Who (EMI), Alphabeat (Universal), Nik & Jay (EMI), Infernal (Border Breakers), Sisse Marie feat. Yes-R (Universal), Clemens (Universal), Ida Corr feat. Fatman Scoop (Lifted Housed)