Here is a tutorial for taking screenshots with your Dreamcast (thanks to Smurph over at DCEmu):
If you don't have a Dreamcast web browser to develop your film after you're done, your in luck.
You can also transfer your file over a BBA or Coder's Cable to your computer and then just develop the file from your computer.
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EDIT: Shortly after I posted this, Smurph made a PSO screenshot FAQ. Here it is:
Thank Smurph from DCEmu everybody.Smurph wrote:Plug a controller into port 4, and a VMU into the second (bottom) slot of that controller. to take a full-screen screenshot, press A+START on that controler. To take a shot of the center of the screen, press X+START on that controller.
For extra fun, plug another controller into port 3. now you can view your teammate's action by holding L+R and pressing UP on the D-pad.
To "develop" your shots, go to:
http://bswirl.kitsunet.org/psossd/index.php?lg=en
with your DC browser. This page will develop your shots into JPG files and you can upload them to your FTP host.
Have fun!