How can a css in a folder read a pics in another folder

I have css in a folder name css. so how can this .css file read a pics file from another folder eg, pics/a.jpg.

Both css folder and pics folder are located in a folder named project.

I know i can call the pic by url(project/pic/a.jpg)

However, its very inconveninet when I change the folder name eg from project to project10.

so, are there any other way to do it.

Please help.

Thanks.