Well, it’s like this. When you see your pictures back on the computer screen (and this applies equally to scanned pics as well as digital) at the ‘normal’ size they probably will look great! But, lurking therein are gremlins that can’t be seen, until you enlarge (or blow up, even) the screen image.
Most software will allow you to inflate the photo by up to, and beyond, 100%. On Adobe products a ‘double left mouse click’ on the hand lens icon will automatically do this. Generally, unless you are doing detailed work, you don’t need greater than 100%. This larger view will then clearly show any spots that appear (muck on sensor), any encroachments around the edge of the frame and any other bits and bobs that you can then get rid of to make your masterpiece even more masterpiece like.
If you don’t already do this, I recommend you blow your photographs up – starting tomorrow. Then watch good become better.