The length doesn't have to be represented by l or L, it's just what was picked for convenience, L is probably the most suitable here. They aren't really equations as such, just intuition. You look at the picture, determine how many wavelengths it contains, and link that to the length. In the first harmonic, the picture shows one half of one wave cycle, hence half a wavelength, the total length is thus just half of the wavelength. The frequency of the harmonics is just general. If you have the 10th harmonic, it's frequency is just 10 times that of the fundamental for example.
Remember that the first harmonic is the fundemental frequency. The reason why the frequency of the others can be related to the fundemental frequency, is that these waves span over the same length, L. As the wavelength of the fundemental is two times the length, with frequency f, if you have one whole wavelength in the picture for the same length L, the frequency is 2f.
For a more rigorous view on this, recall this equation:
v=fλThe velocity in any harmonics is a constant, hence
fλ is a constant. In the second harmonic, your wavelength is equal to L, hence, half of the fundamentals wavelength. For
fλ to be a constant, the frequency must double.