Strive only provides images for most of the unique slaves acquired through quests, and it does not provide any images for random slaves, which is why there is a section on itch.io (https://itch.io/board/252654/portarits-packs) and Discord(https://itch.io/t/284398/discord) for images. Since Emily's portrait appears I believe we can conclude that the problem is unlikely to be a hardware issue.
Strive runs on the Godot engine which only supports loading images with ".png", ".jpg", and ".webp" extensions. The slave image selection scripts contain filters to restrict the displayed files to exactly these types, which has the side effect of excluding game images from being selected for slaves.
The "Add custom image" button allows the user to change the file type filters and select images with other extensions, but that doesn't mean that the game will be able to use them. Additionally, that button does not provide nor maintain the tags of the image, so it will not be compatible with the "Race locked" option nor the automatic random portrait assignment system.
I recommend grabbing a portrait pack from the itch.io section linked above and extracting it to the location mentioned in this thread https://itch.io/t/3797794/struggling-to-install-portrait-packs#post-10016618. If that does not work after restarting the game, then I recommend grabbing the Debug mod to check for other problems (https://itch.io/t/1137280/debugmod-v10d. Note the game won't automatically assign images to existing slaves without images in a save file unless you get the Bugfix mod (https://itch.io/t/1167355/bugfix-for-10d-v6b), though it won't fix your other problems.