mount

Get the partition layout of the image

$ sudo fdisk -lu sda.img
...
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
...
  Device Boot      Start         End      Blocks   Id  System
sda.img1   *          56     6400000     3199972+   c  W95 FAT32 (LBA)

Calculate the offset from the start of the image to the partition start

Sector size * Start = (in the case) 512 * 56 = 28672