The difference is same as of a Folder in Windows.. If you are placing all the images in a folder, the view gets different and it uses Thumbnails view..
The same is for Picture Libary/Images App in SharePoint.. It will give you thumbnail view of the images.. Picture Library is actually an enhancement to the Document Library..
Also one of the important usage of Picture Library is that it creates usable sizes/optimized images Out of the box.. So let's say the user uploads image of 5760 x 3840 size.. But when you want to use this image within your Web Part or other places in site, you will need optimal size (so that the page doesn't take too long just to render the image). For this, three different sizes are stored in SharePoint Picture Library:
AFAIK, This isn't available in Document Library