If you are trying to label two similar images in sequence as a and b, then the most effective method is to use a thing called field specific switches.  

The basic method:

1. Add your first caption
2. Add the next image and second caption.
3. Right-click on the number of the second caption and select Edit Field.
4.  With the Field Name "Seq" selected, press the Options button in the bottom left.
   
5.  Click the tab that says, "Field Specific Switches" and select the \r Switch.
6.  Press the Add to Field button on the right.
7.  Now place your cursor in the Field codes entry box in the center.
8. Type whatever number you wish to reset to. (For example if your figure is Figure 4a, then type a 4 for Figure 4b.)
   
9. Now you can hit Ok in this menu and the previous one to have the field changed in your document.
10. Type any letters you wish to add after the sequential numbers in your caption.
11. Be sure to update your List of Figures/Tables before saving, to check that they are working properly.

This method also works if you want to label Appendix or Supplemental captions with prefixes such as Supplemental Table B1. You can reset the sequential numbering for each Appendix section in the caption and then add the respective letter before the caption number.