Correct me if I'm wrong as I have never used veracrypt before.

You said that you used an image downloaded from pixiv as the key. Is it possible the file on pixiv have been modified between the first and second time you downloaded it?
Maybe the artist updated the post with a new image. Maybe pixiv has messed with the file in some way (like that time when they removed EXIF data from existing images https://github.com/Nandaka/PixivUtil2/issues/807)

If that's the why it doesn't decrypt, maybe you can try your luck reverse search the image and see if you can find the original image on any boorus or anywhere on the net.

I think an image on gelbooru or danbooru would be a better option for this use case, since you can search for images with a md5. You can write down the md5 somewhere so you can be sure you're using the same image.