The images are simple connected graphs that represent digits from 0 to 9 in the 24-hour time. Here they are in order:

You might get a better idea of the rationale for ordering them this way by looking at the equivalent graphs in the lower row. I could have drawn them this way, but I chose the upper renderings. The upper and lower versions are the same graph!

There is some easily found property of the graph that is monotonically increasing from digit to digit; moreover there do not exist any graphs with intermediate values — that is, we didn’t “skip” any.