Change webpages' background to the picture you choose. Press F2 to change the picture, F4 to change the opacity of the foreground elements.(根据你选择的图片更换网页的背景，按F2更换背景，按F4改变前景元素的透明度。)
Because of github's security policy, the background can't display on github.因为github的网页设置导致GitHub上背景图片显示不出来。
After zooming the page, you have to flush the page to make the background image fix the screen.缩放了浏览器以后得点刷新才能让背景图片正常显示。
You should input the origin url of image in the F2 popup menu. You can right click the image and click 'copy the image link' and then paste. Because of the limitation of the explorer, this script doesn't support local image.注意图片选择框里要填的是图片的原始url，可以右键点击图片然后点“复制图片链接”再粘贴，由于浏览器的限制，本脚本不支持本地图片。
