There is a use of 'to charge' that means 'to load' or 'to fill'. Today it is mostly used in relation to loading a battery. You 'charge' a battery. 'I must charge my phone battery.' 'My phone battery is charged.' So 'politically charged' means 'loaded with politics'. 'emotionally charged' means 'loaded with emotion'. This use originally comes from French. The French word for 'to load' is 'charger'. 'Charge your glasses' is an old fashioned way to say 'fill your glasses' (to make a toast with Champagne at a wedding for example)

Meaning that the graffiti content was about politics. The word is just to highlight the nature of the noun. Another example would be "emotionally-charged" meaning with a lot of emotions. So in this case, with a lot of political content. Hope this helps!