The CSS background position can be set with keywords, length, and percentage values. The keywords are pretty self-defining. For example, background-position: top; places the background at the top of the container. The keyword values are: top, bottom, left, right, and center. These keywords can be combined with offset values to adjust the position (more below). Further, percentage values can be used to position the background. background-position: 25% 75% positions the background 25% from the left and 75% from the top. Finally, it is possible to define the position with length values. background-position: 20px 30px will position the background 20px from the left and 30px from the top.
There are a few values for background attachment including fixed, local, and scroll. The fixed value will fix the background to the viewport. It will not scroll with the element. The scroll will allow the background to scroll when the element scrolls. The local value will fix the background to the element's contents. If the element can scroll, the background will scroll with its contents.
There are several ways the background size can be defined in css. The contain keyword scales the background as large as the container it is in without changing the size of the image. If the image is smaller than the container, it will repeat in a tiling pattern unless background-repeat: no-repeat is specified. The cover value will scale the image (while maintaining the ratio) to the smallest possible size where both the width and height cover the entire container. When one value is assigned, the value defines the width of the image and the height is automatically adjusted to preserve the ratio. When two values are assigned, the first value is the width of the image and the second is the height.
The edge offset value is set on the background-position property. The offset values can be attached to the top, bottom, left, and right values. The left/right define the X value, and the top/bottom define Y. The value that comes immediately after the keyword is the offset value. For instance, background-position: bottom 50px right 100px; will set the background 50px up from the bottom and 100px from the right border.
When multiple backgrounds are used, they are stacked on one another in the order they are defined. If, for example, there are 3 background images, the first listed will appear on top, the second will be immediately underneath, and so forth. When defining sub-properties of the background, the listed values follow the same order as the background images. The first listed property applies to the first listed background, and so on.
There are many ways to style a CSS background, and these are just a few of the basics. It is important to understand the different changes that can be made to the CSS background to get the look you really want. More information can be found in the Mozilla CSS Guide.