How can I create a new asset source? Now I have a "gallery image" field which allows users to upload multiple images, and some "single-upload image field" in matrix block. 

For "single-upload image field", I want to force users to choose images only from the "gallery image" field.

To do that, I want to create a custom asset source called "gallery image". So with this source selected, I can limit the source of "single-upload image field".

Any idea to achieve this? Thanks!
Of course, just create a new asset source (settings > volumes > new volume) 
then in your image field select 'Restrict uploads to a single folder' in your field and then select the asset source you just created.