Encode your image with openssl

openssl base64 -in rectangle-520-copie.png | tr -d "\n" 

Use in your stylesheet

background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAABmJLR0QA/wD/AP+gvaeTAAAACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH4QMVCSA4g/wYSQAAADlJREFUOMvtzEERADAIA8GjUyv41wOIoY9ayDMrYKOqNiJQ2F0uQGZKwu7mIObQoUOHDr8LMDOy8AHxlAxEmykZzwAAAABJRU5ErkJggg==);