All pictures are in the public domain, either extracted via the filter “free to use, share or modify, even commercially” of the usage rights option in the advanced search settings on Google - or from specifically public domain websites (e.g. pxhere.com).
Some Wikimedia Commons images do require citation however - these are those pictures I have used whose authors requested citation:
Megistias:
https://commons.wikimedia.org/wiki/File:ExpansionOfMacedon.jpg
Robert Howlet:
https://commons.wikimedia.org/wiki/File:Isambard_Kingdom_Brunel_preparing_the_launch_of_%27The_Great_Eastern_by_Robert_Howlett_crop.jpg
DragonTiger23:
https://commons.wikimedia.org/wiki/File:3_-Murad_I_map.PNG
Andrew Dunn:
https://commons.wikimedia.org/wiki/File:AlexanderTheGreat_Bust.jpg
GenericMaps:
https://commons.wikimedia.org/wiki/File:MacedonEmpire.jpg
Sailko:
https://commons.wikimedia.org/wiki/File:Medaglioni_aurei_romani_da_tesoro_di_aboukir,_inv._2426_alessandro_con_diadema.jpg
Luis García Zaqarbal:
https://commons.wikimedia.org/wiki/File:Batalla_de_Gaugamela_(M.A.N._Inv.1980-60-1)_03.jpg
stefg74:
https://commons.wikimedia.org/wiki/File:Mytikas.jpg
Giovanni Dall'Orto:
https://commons.wikimedia.org/wiki/File:Istanbul_-_Museo_archeol._-_Alessandro_Magno_(firmata_Menas)_-_sec._III_a.C._-_da_Magnesia_-_Foto_G._Dall%27Orto_28-5-2006_b-n.jpg
Neithsabes:
https://commons.wikimedia.org/wiki/File:Louxor_Amon_Ra_Alexandre.jpg
Sound clips/audio all from public domain sources as well (here on YouTube).