I suffered from this problem around a year ago and decided to look for a solution, I found this forum (guide link), which explained the process. This should aid in your problem.
Here is the custom cursors I created and use, I find it much easier for me to keep track of (see attached)
There are versions included that don't have the pink and yellow star as well.
and here is a link to the guide I used, the do-it-for-you application no longer works as Riot changed their file format. However the Resource Hacker guide (#3) still works. Caution: When I did this about a year ago, I ended up having to reinstall sometimes when they patched, regardless of whether I replaced the cursors with the defaults. However the patcher was known to be awful at this time.
Link to the guide:
Link to resource hacker:
Pictures attached, cursors I made as well as defaults inside the attached zip.
The directory that contains League of Legends.exe for where you navigate resourcehacker should be something like:
C:\Riot Games\League of Legends\RADS\solutions\lol_game_client_sln\release s\0.0.0.100\deploy
Hope this helps,
edit: re-added zipped .cur files that I modified as well as the originals. Also included a picture of what it looks like out of game.
Also to the person who posted below me, Cabadath, I modified this and made this post a long time ago, I know you can't make it bigger. I'm not sure about smaller, at any rate if you wanted to use a smaller cursor you could use the same size background but fill in with more clear pixels.
EDIT: THIS METHOD NO LONGER WORKS, I've posted a new method on page 5.