Taking a photo is not something I am often asked to do on a shoot, although recently I did a job where the client wanted a few publicity stills of the video. There are two ways to do this on the C300.
1) In video mode. Assign a button to "photo" and then click away whilst recording video. The photos are stored on the SD card not the CF card.
2) In media mode. Assign a button to "photo". Pause the video and press the assigned photo button. As in video mode photos are saved to the SD card.
NB I tried to use the same button I had assigned when in video mode and it didn't work, so I then assigned a button in media mode on the top of the camera where many of the media mode buttons are, like play, pause etc are and this worked).
Using the 2nd method is a great way of getting decent shots, having the choice of multiple frames is great as you never need to take photos when someone is blinking, grimacing etc and you can just role on to the next frame and take your picture. These pictures are not the highest resolution and tend to be around the 1MB mark. However this is plenty for the web, when blowing them up full screen on my iMac, the pictures looked great.