To take a picture, press the optical joystick. The camera will take a picture. Then press the Done button, and the picture will be uploaded to your photo hosting service automatically and a link to it will be inserted in the tweet.
Now, press the Globe icon. You'll get a Post Your Location menu. It gives your latitude and longitude and accuracy (mine was 24 meters via GPS). You can Update GPS Now, which redoes the GPS fix. If you do this, you'll need to press the Globe icon again to get the menu back.
The other two options are Insert Maplink, which puts a link to a Google Map into the tweet, and Insert Location Name, which inserts the location. Again, if you use either of these options, you'll need to press the Globe icon again to get the menu back for any others.
Now, you can add any text you want to the tweet. When I did this, I inserted the street address and ended up with 52 characters left. You should be fine with just the map link. Once you've composed the rest of the text for the tweet, press Update and Peep will send the tweet out!
Yes, it really is that simple with the Droid Incredible! For citizen journalists and volunteer workers, this seems to me to be the device of choice, at least where Verizon coverage is available. If you want to contribute setup instructions for other devices, please feel free to post them here.