Why we should use diaphragm to singing? i've heard that the diaphragm force the oxygen to lungs? is it true?

Your diaphragm is important for singing, because it hold air in the body, which allows the singer to let out and control the notes they sing, whether high or low, while singing without putting little to no strain on their throat.

well seeing i am an artist and i actually did a self portrait; i will describe how i painted it.   i used acrylic on a stretch canvas about 4 feet wide by 6 feet tall. i painted myself smiling in a soft pink long dress, white tile floor sitting on a white couch with a table behind and a vase of gorgeous flowers.   i also had a round white hat with a soft pink ribbon through it laying on the floor and on the table behind me was a lit candle. i kept it simple but soft and romantic.