Welcome to LQ!!
So far, you may have received some incorrect or misleading answers.
First, I assume that your "img" file is an image of a disk partition--or of a whole disk. Please confirm this and tell us how the file was made.
Also, please give details on what does not work correctly....
When you use dd to copy something onto a drive, you do not need to have the drive partitioned or formatted---dd is a byte level copy and will duplicate exactly what the low-level data was on the source device (or image file).
It is important to note the difference between copying a disk image and a partition image. The disk image will include the partition table, and the partition image will not. Thus, if your img file was from a partition, and you copied to the device (sda), you would wind up with no partition table.
Finally, what do you mean by the "operating system for the card"? From the context, it sounds like you meant filesystem, which is what gets installed when you format a drive or partition.