The Still numbering does this sorting automatically.
From the manual.
Stills display an identification number in the upper left corner, that consists of three numbers separated
by periods:
The first number is the video track it’s saved from.
The second number is the number of the clip the still is from.
The third number is incremented every time you save another still from that same clip.
For example, the third still that’s saved from clip 4 in track V1 is numbered 1.4.3.
Peter