i need to add a caption to an iphone or ipad photo for marketing the item.

Nice Flower!

Are you asking about drawing or typing on the picture? That's called Markup- here's some info

Use Markup on your iPhone, iPad, or iPod touch - Apple Support

I used Markup to add this "Happy Days" caption to an old picture.

There is also a Caption field, where I put what was written on the back of the picture, but that doesn't show up on the picture, itself--just when you ask open the info window.

I wasn't sure, but is this what you were asking about?

Janesjazz wrote:

doesnt work on ipad

so NOT helpful!!!!

But the picture I posted I did on an iPad right then. When you choose text, you have the option of using the iPad’s keyboard, or even writing by hand and having it convert.

To illustrate Richard's solution here's ;where you find Markup after entering the Edit mod:

Once you're in Markup you add a text box with this: