How do I create a button that enables clients to place a photo from their computer ?

There is, but not if it has to work with Reader. For more information, see: http://forums.adobe.com/thread/743823

Please to add your voice to that discussion to increase the chances of this being added to Reader in the future. I have hope that they can be convinced to do this.

If you need to have this today, you can create an XFA form in LiveCycle Designer and use an image field. The alternatives for a form created in Acrobat is to have them attach the image as a file attachment comment or somehow get the user to copy the image to the clipboard and paste the image as a stamp. The former won't display on the form and the latter won't necessarily be placed or sized how you intended.

I have a photo documentation form that is used with Adobe Readers all the time.  Adobe Reader's have some security features enabled to protect the users from malious code and disables any code in a PDF.  To disable this, open Adobe Reader, Click on Edit and select Preferences.  Under the General category, deselect the "Enable Protected Mode at stateup", then click OK.  Only disable this feature if you trust the author of the PDF.

And then, how do i add a photo in the pdf file?

You have to use LiveCycle Designer or a LiveCycle Server product to create a form and enable special rights. Follow the link posted above or:

http://forums.adobe.com/thread/743823

It's true that a form created in LiveCycle Designer can include an image field that works with Reader, but usage rights have no relevance here. The buttonImportIcon method doesn't work with Reader whether the document has full usage rights or not, unless Adobe decides to make a change with a future version.