This should be in Modding Questions and not Modding Discussion

You can alter the portraits with any image editing program that will open .tga files. If you are using Rome Total War you won't be able to see the portraits until you unpack the UI files from the .paks. For BI some portraits are visible in ui/culture/portraits file. Note that there are 6 portraits for each general, 2 different sizes, young, old and dead. If you alter one you should alter the same number file in the other folders so he is consistent.

In RTW and BI you can not assign a specific portrait to a specific general but the game does seem to always use them in a pre-set order. so if you can work out which file is being used for the first general in descr_strat you can change that portrait.