[color="Blue"]I took the liberty of merging your two threads about this, since the purpose is quite similar

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In general, you find the portraits you wish to replace (usually located in the "Graphics\Units" and "Graphics\Armies&Fleets"-directory within your installation*), and put the pictures you want to replace them with in their place, using the same filenames as the originals, and adherring to the same imagesize and format as the original.
Note that pictures for any given unit/leader comes in pairs; one for use in the unit bar and one for use on the map, which is why you need to think about 2 directories in the above procedure.
[SIZE="1"]*This is were they are in AACW, at least[/size]