The procedure for putting MP3 ringtones on this phone is the simplest system I've come across. No BitPim / QPST / etc is required.

First install the LG drivers on the PC, see As of this writing, the Cosmos 2 / VN251 is not listed, but the driver shown for the vast majority of LG feature phones worked for me. (I have Windows 7 64-bit.)

Set the phone's USB mode to "mass storage": Menu, Settings & Tools, USB Mode, Mass Storage (or "Ask on Plug").

Connect the phone to the PC. If the phone asks for a connection type, select "Mass Storage". Wait for the new hardware to be detected; wait until a new drive letter appears in MyComputer or Windows Explorer.

Open that new drive, and open the Ringtones folder. Drag your MP3 ringtones into this folder.

That's all there is to it. On the phone, go to Menu... Media Center... Music & Tones... My Ringtones, and your MP3 ringtones should be there in the list ready to select. I didn't even have to reboot the phone.