Pictures are named DCP_nnnn.JPG, and are numbered in the order in which they are taken. The first picture is named DCP_0001.JPG; the highest number allowed is DCP_9999.JPG.
When a Picture Card contains a picture named DCP_9999.JPG in the \DCIM\100K3900 folder, subsequent pictures appear in a new folder (\DCIM\101K3900), beginning with picture DCP_0001.JPG.
Even if you transfer pictures to the computer or delete pictures from the camera, the camera continues to use consecutive numbers for all subsequent pictures. For example, if the last picture taken was DCP_0007.JPG and you delete it, the next picture you take is named DCP_0008.JPG.
When you insert a different Picture Card into your camera, the next picture number is defined by incrementing the higher of the last picture taken by the camera, or the highest existing picture number in the folder.
If you use the Picture Card in a camera other than the DX3900, and that camera manufacturer adheres to this file structure standard, the \DCIM folder contains a folder named by that camera. For more information, refer to the documentation that accompanied that camera.