Missing photograph

The missing photograph is one of:

Werner von Siemens, the scientist after whom the SI unit siemens is named.

To work this out, first identify the pictured people. These are all:

Scientists who have units named after them:

Charles-Augustin de Coulomb
Joseph Henry
Georg Ohm
Nikola Tesla
Alessandro Volta

Each of these units has a single-character symbol: Coulomb (C), Henry (H), Ohm (Ω), Tesla (T), Volt (V). Now to generate the second row of images, imagine we replace the images with the corresponding unit symbols, and reflect these characters in a horizontal line beneath them...

Whilst reflecting the 'T' and 'V' gives 'an upside-down T' and 'an upside-down V' (explaining why the corresponding photographs of scientists are shown reflected), the 'C' and 'H' (being horizontally symmetrical) are unchanged, thus Coulomb's and Henry's pictures are shown the same way up as before in the second row.

This means we need to identify...

...a scientist after whom a unit was named, whose symbol is an upside-down omega (i.e. the symbol for Ohm, reflected horizontally): ℧.

This would be:

Werner von Siemens, as the siemens unit (whose symbol is traditionally a capital 'S') has also historically been named the mho - 'ohm' backwards, since the ohm and the siemens represent properties that are the reciprocal of one another: electrical resistance and electrical conductance - and the symbol of the mho is the upside-down capital omega we require.