Selecting a Firmware Image
If you are flashing from the factory firmware, download the firmware that has 'factory' in the name. If you are upgrading from a previous version, download the firmware with 'sysupgrade' in its name.
Note, however, that there are two factory images created for the WNR2200. One has 'NA' on the end, and the other doesn't. From my research, the 'NA' stands for 'North America'. However, it also seems that Netgear did not make a different firmware for North Americans. Not being from North America, I didn't care about this and just flashed the one without 'NA' in its name. If you are from North America, then you should probably contact someone on IRC, the mailing lists or the forums to get a more definite answer. I also saw that Netgear listed on their website that there are firmware differences between the worldwide versions and the Russian and Chinese versions, so if you're from Russia or China, you might want to ask someone before flashing as well.
If you are unsure of which version to flash, do not flash. You could end up bricking your router if you flash the wrong version
Flashing from factory firmware
On the WNR2200, Netgear have made it such that you can't flash OpenWRT through their web interface, so to flash (without a serial console), you'll have to use tftp and recovery mode. So, let's get started.
put <TYPE FIRMWARE FILE NAME HERE>
Once you've done all of this, wait 5-10 minutes and then reconnect to your router. Do NOT touch the router during this time. If all goes well, you should be able to access your router by the command