[Serializable]
public class Illustration : ISerializable
{
    public Texture2D Image = new Texture2D(256, 256);

    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        info.AddValue(nameof(Image), Image.EncodeToPNG(), typeof(byte[]));
    }

    private Illustration(SerializationInfo info, StreamingContext context)
    {
        Image.LoadImage(info.GetValue(nameof(Image), typeof(byte[])) as byte[]);
    }
}

[Serializable]
public class CustomAnimation : ISerializable
{
    public Texture2D[] Images;

    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        info.AddValue(nameof(Images), Images.Select(x => x.EncodeToPNG()).ToArray(), typeof(byte[][]));
    }

    private CustomAnimation(SerializationInfo info, StreamingContext context)
    {
        var textures = info.GetValue(nameof(Images), typeof(byte[][])) as byte[][];
        if (textures != null)
        {
            var imagesAndTextures = Images.Zip(textures, (i, t) => new {Images = i, textures = t});
            foreach (var it in imagesAndTextures)
            {
                it.Images.LoadImage(it.textures);
            }
        }
    }
}

"Illustrations": [
                {
                    "Image": {
                        "instanceID": 34540
                    }
                }
]