Time stamp on the photos?

No, there is not. You can see the time & date when you import your photos into your photo editing software.

A time stamp is embedded into the EXIF data of every photo you take with your iPhone. You can view this in any EXIF compliant image viewer.

As I said in my original reply to you, you cannot.

is it possible to place a visual stamp right on the image itself?

Not on the iPhone itself, but you can do it on your computer using third-party software, such as:

http://www.batchphoto.com/howto/add-date/add-date.html

There are a number of camera apps that can do this. Some are paid and some are free. Put both the time and date on photo itself and some even let you change the color of the text. Just search the app store and you should be able to find one that does what you want.