My iphone assigns random numbers to my pictures instead of sequential

So for example, today I took three pictures, and the iPhone labeled them IMG_2157, IMG_3754, and IMG_4731.

Another curiosity, sometimes the pictures are .jpg, and sometimes .JPG (upper case).

None of this seems right or consistent, which one would expect from a computer.

As you can understand, this has caused some confusion on import. How can I set the phone to make the picture numbers sequential again?

iphone 4, 7.1.2