Its possible to get banned for this, but there is like 1% chance for that, or even less, i have been using custom icons for like a year and still playing (My main is legit, never used any hacks/cheats on it) i made a ticket about custom icons a while ago just because people asked me if they are bannable or so on. thats what i got:
Hello,
Thank you for writing in. Modifying your game files is something that we do not recommend as it might trigger the anti-cheat system to flag your account. You may do this, but please understand that you are risking your account by doing this.