By backup I mean creating an image file that is shareable over the internet, not making clone sd cards. Thanks.
What we normally call an "image" file is a clone of a SD-card , mem-stick or CD/DVD-disk.
Good old Norton Ghost had features where it saved partition information and then also understood the filesystem so it only collected the files. So you never stored unused space.
"Partimage" and "Clonezilla" might be worth looking at.
There is a tool called "zerofree" that writes zeroes in all the free blocks on the linux filesystem, that means that when you compress a clone image later it will pack much better.