Why do the overlay icons sometimes change to random graphics?

The Windows icon cache is a fairly buggy creature. You can solve this in one of the following ways:

• Install Microsoft's TweakUI and run the option to rebuild icons.
• Or increase the icon cache size. Go to HKEY_LOCAL_MACHINE\\Software\\Microsoft\\Windows\\CurrentVersion\\Explorer and add a new String Value called Max Cached Icons. The default value is 500 - try increasing it to 2048 (see Q132668 in the Microsoft knowledge base for more details).
• Or delete the file called ShellIconCache in your Windows directory. And reboot.
• With TortoiseSVN 1.3.0 and later, you can also rebuild the icon cache by calling TortoiseProc from the command line like this TortoiseProc.exe /command:rebuildiconcache