I have my both my Article and Media categories set the same way like this image shows below. So if I added an article in the Year 2018 sub category - I would also add any images used in that article in the Media year 2018 sub category. But I have noticed a problem with images uploaded to sub categories not staying in the right one they was uploaded to, instead moving themselves into the first Year 2017 sub category when so many images have been uploaded already into the 2018 one. Like an overflow effect is happening and they start being auto moved into the first Year 2017 category when reaching a set amount.
So I have now had to delete any sub categories used with MEDIA and just use one category only to store them all in. As it seems it's pointless using sub categories with uploaded media items? If it's just going to randomly move images to another years sub category that you didn't actually uploaded the images to in the first place.
And I have also noticed. If I went into the Media section and searched all images in the Year 2017 sub category, then spotted some articles images used for Year 2018 in it. Then click edit the media item and choose the 2018 sub category to move it into instead. That doesn't work either and it stays put in the Year 2017 sub category still.