If you don't want to use a 3D model, you can use a 2D portrait instead.
By default, the mod comes with a default error (colorable and non-colorable) portrait. Creating your own portrait is actually simple!
You need to create a file in the data/tf2_hud/minimodel/ directory, and name it <your_own_name>.plyimg

Paste this template into the file:

Set the key value layer1 to the path of your image (Path starts at "materials/tf2_hud/minimodel"). If you want to color your portrait, you need to split it into multiple images. Make the colorable images black and white, and separate non-colorable parts of the portrait.
In order for the layer to be colored, change the value of layer_color corresponding to the number of the layer from 0 to 1.