Just go into this folder: C:\Program Files (x86)\Steam\steamapps\common\Shadowrun Returns\Shadowrun_Data\StreamingAssets\ContentPacks\seattle\art\portraits
and substitute any portrait file in there with what you want your portrait to be.

***Make sure the portrait you want to use is 212x278 pixels and PNG format.

Here's another shot of it working in Character Creation from my Steam screenshots Example , though you can change it at anytime you want.