Deleting photos is covered on page 88 of your manual.
Although the manual does not say it, I would assume that you would have to remove the memory card before you can delete a photo in the internal memory:
Put the camera in the Playback Mode
Use the 4-way controller to choose an image
Press the trashcan button
Use the 4-way controller Up button to select Delete
Press the OK button.
If that does not work....you can format the internal memory.
That is on page 108 of your manual.
That will delete ALL photos in internal memory:
Remove the memory card.
WARNING, if you do not remove the memory card you will format the memory card and all photos will be erased.
Go to the setup menu and select Format
Use the 4-way right button
Use the 4-way up/down buttons to select Format
Press the OK button
Wait until it completes.
I have an Pentax Optio S40 camera. I have some pictures on the camera memory(not the memorycard) that I can not seem to delete. Can anyone help me?