Possible Custom Portraits

[Chocolys]

I highly doubt they will implement custom paintings because:

1 (the main big reason): They would need to store on the servers every custom painting picture created by the players, so that would require server resources. (And you know how SE is about its server resources )
2: Each time any player enters the house, His game client would have to download the custom painting picture dynamically from the server, as obviously you can't practically have every custom painting created by every players installed on every game client. Eventually the player's game client could cache the pictures as to not have do redownload them, but still not very neet.

However what they could implement Character portrait, either by using a similar method they use on Lodestone/Forum for your character portrait, or by using the same technique used for placing Retainer inside a house, (excepted it will be used to render a flat picture and not a 3D model). But in both cases the picture would not be from a manual screenshot you took but would just be generated from stored or current character appearance data.