How do you delete all pictures simutaneously from Nikon L18 camera?

The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.

Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive, the same way you copy any other files. Or you can use Nikon Transfer or any other photo cataloging program.
You can download the current version of all (free) Nikon software here.

The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.

Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive. Or you can use any photo cataloging program, such as Nikon Transfer.

Are you using the Nikon software? It should be able to see the picture. Also, the camera has both a memory card and internal memory. If the picture is on the internal memory and a card is inserted, the picture will not show up. Remove the card.

The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.

Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive, or you can use software like Nikon Transfer, available along with all current Nikon software at http://support.nikontech.com/app/answers/detail/a_id/61

Consider NOT connecting your camera to your computer.

The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.

Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive, the same way you copy any other files. Or you can use Nikon Transfer or any other photo cataloging program.

The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.

Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive. Or you can use Nikon Transfer, available along with the current versions of all (free) Nikon software, from http://support.nikontech.com/app/answers/detail/a_id/61

Transfer them to a computer then delete them.

I noticed that, after dumping the full SD card onto my computer, I didn't seat it perfectly back in the camera (my finger bottomed out on the batteries before the memory card 'clicked' into place. With the camera unable to see the SD card, it defaulted to the internal memory, which is very small.

I got the SD card seated fully and the "Out of Memory" message went away.

I suggest you to remove the card from the camera and use a memory card reader to transfer the pictures to the computer. Note that the memory card should be formatted in the camera. Once the pictures are transferred you can reinsert the memory card to the camera and then format it in the camera.

There is an alternate way of downloading images from the memory card from the camera.
Get a USB memory card reader, remove the memory card from the camera, insert it into the card reader, then plug the card reader into the USB port of your computer. You will now see card reader as an external memory device, open the folder and copy your photos onto your hard drive.