I would recommend downloading the mod
Portraiture. It allows you to just keep folders of png's for your portraits under your Mods folder rather than using the xnb files. Because its a png its "unpacked" and needs to be packed. That can get complicated so Portraiture is much more convenient c:
Also I'm not sure which Portrait mod you are trying to install but just moving the whole Content Patcher mod into your mods folder with SMAPI is how you would run that.
hope i could help !