Pictures are taken by presing 'Up' on the d-pad. Your screen will change, depicting a negative image and a shutter (kind of... =/). The photo will automatically attempt to upload, as shown by the text on the right side of your screen. Three successful uploads will net you painless 15G.
The content of the photograph is irrelevant, but they *must be taken in a RANKED match*.
Users who experience a failed upload should first try another game. If still no cigar, you may need to adjust network settings (check your NAT).
*Thanks to Spilner1001 for confirmation on this.