You can add your own if you want you just have to use the right naming convention for the images and put them in a correctly formatted folder. For example Catura's ID is 7 and the game internally reads her name as シャトラ so if you wanted to add an image to her you'd make a folder named 7シャトラ(you can find id number by their csv file and the Japanese name the folder requires in CHARNAMES_TR.ERB).
The only thing that's kinda annoying/tedious is figuring out the naming format for images there's a txt file in the resource folder that explains it but mtl makes it kinda hard to understand at least for me. But for example if you wanted to add a default image just so they didn't get the blank white square you'd name the image 短冊_デフォルト and that would be used in any situation where they had no appropriate images for other outfits or positions.
But if for example you wanted to add alternates for like when they're wearing a swimsuit you'd name it 短冊_水着 and if you wanted to add variations to either the default outfit or swimsuit you'd name it 短冊_デフォルト_差分_表情 for the default outfit and 短冊_水着_差分_表情 for the swimsuit. If you wanted to add even more variants it seems you'd name them with the suffix _差分_表情 like in the above but just add 1, 2, 3 and so on at the very end so it'd look like 短冊_デフォルト_差分_表情1 for example.