The "problem" is that with most units the portrait is actually in a seperate file, if you look in the MPQ at Footman.mdx for instance you'll notice there is also a Footman_portrait.mdx in the same path as the footman (units\human\footman\ for example) so you'll need to extract the matching portrait file for whatever unit it is you're having trouble with and make sure it has the exact same name as the unit model followed by "_portrait.mdx" (minus the quotation marks)
Note there are some models (specifically buildings) which don't have portraits, this is because they have a built in portrait effectively but most have a seperate model. Hope this helps you both :D | 
