Hello Mellysa
Sometimes, when an img is in PNG format, it have some properties that matlab change, such as transparencies. I recommend you to change it to .JPG format, and perhaps re-colour your backgound to avoid problems when reading the image.
About accuracy, if black-borders are touching your kanjis, as is happening in the first image, the CNN may contemplate it as part of your symbol. but, as i can see, every single image have exact the same black-border, so, i think the network may "erase" this border when using ReLU layer. Also, you can train your network to test if borders affect, and perhaps check the activations.
