Imagine you have an 8 GB harddrive with two partitions. The layout might look like this:
0 2016 3904992 15624000
+--+--------------------+----------------------------------------------------+
|PT| System-Partition | Work-Partition |
+--+--------------------+----------------------------------------------------+
PT = Partiton Table
If you only image the System partition the image file will contain this:
0 3902976
+--------------------+
| System-Partition |
+--------------------+
And if you image only the Work partition the image file will be like this:
0 11719008
+----------------------------------------------------+
| Work-Partition |
+----------------------------------------------------+
On the harddrive the partitions started at sector 2016 resp. 3904992. In the image files they start at the beginning of each file.