Sharepoint - What is the difference between Document Library, Picture Library & Asset Libray?

I am not going in detail. The basic difference between these depends on your primary requirements.

If you want to upload image and want to show a slide show on any site pages then in that case you have to use Picture Library because other 2 does not have these option.

If your purpose is just to store image then in that case you can go with Asset Library

If you want to maintain the Check-in and Check-out for your pictures to then you should go with Document Library.

These are basic difference you asked for. Each and every library is unique at its own position. You have to choose amongst them depending on your primary requirement.

Let me know if you need more clarification.

If you store images in a document library, when you search for said images in the site, they will not show you the thumbnail, you will be shown the icon instead. Images inserted in a picture library will display the thumbnail.

The picture library has some built in views specific for images that other libraries do not have.

Asset library can technically contain any type of file that does not need versioning or approval. It is usually the place where the files you use in a blog, wiki site or article pages are stored. Example: if you write a blog or an article page or a wiki page with text and images (or movies, sounds or any other type of doc, most of the times the site assets library is presented to you as a place to store all those).

The document library is to be used for standard documents that potentially need its standard features (check in/out, versioning and approval).

All previous answers are true. Just to clarify, the difference between the libraries actually lies in the content type on which they are based. For example, a Document library is based on the Document content type. Actually, you can turn a library into a different type of library by enabling management of content types in Library Settings and then adding different content types to the library (image, video, audio, etc.).