How to add alternate text to selected image?

Dear All,

I want to add aleternate text to selected image(jpeg,bitmap,png,tiff).Is this possible by creating plugin?

    PDPage page = NULL;

    //Create a PDPage object

    AVDoc avDoc = AVAppGetActiveDoc();

    PDDoc myPDDoc = AVDocGetPDDoc(avDoc);

    page = PDDocAcquirePage(myPDDoc, 0);

    //Get selection on the page

     if(ASAtomFromString("Image") == AVDocGetSelectionType(avDoc))

     {

         void *selection = AVDocGetSelection(avDoc);

        if(*selection != NULL){

              PDXObject xObj = static_cast<PDXObject>(AVDocGetSelection(avDoc));

        }

     }

now ,how can i add alternate text to selected image. Any help would be greatly appreciated.

Thanks