sure thing, use GIMP & the mod tool using this tutorial, and you will also want
REDitor II (free version will work fine)
create the portrait, save as DXT5 dds
load the portraits and portraits_hi iff files in the mod tool, find a player to replace
use this resource (check both portraits and portraits_hi listings)
viewtopic.php?p=1432583#p1432583figure out what 4 number ID you want to replace, and replace that portrait in both files
then using the REDitor, find the player you want to give that portrait, and enter in that 4 digit ID in the portID column