How to add the picture library in Communication site?

There is no picture library per se, but you can do the following. It just depends on what you want to do with the images.

• Option 1: Create a document library and put pictures in it - call it Pictures if you like! Add the Picture content type to the Doc lib and make it the default so you can see the digital image metadata. Use the Tiles view to see thumbnails of the images or surface the images in an Image Gallery web part on a page. 
• Option 2: Create an Image Gallery web part and add your images to the gallery. You can give the images a title and a caption as you upload them. This creates a Folder called Image Gallery (followed by a bunch of characters) as a sub-folder of the Site Pages library in the Site Assets library.
• Option 3: If the purpose of adding the images is about having a fixed library to use on pages, you can also pre-upload images to a folder or directly to the Site Assets library. That's the default storage space for all images used on modern pages. The images on a page land in a Folder that is dynamically created (along with the Site Assets library) when you use your first image on a page. Each page gets a Folder with the same name as the name of the page (but if you update the page name, the folder name does not change). So the path for page images is Site Assets  (library)> Site Pages (folder) > Name of Page (folder).