If you have a lot of images in your gallery, you should enable the Pagination Thumbnail Creation parameter in Global Configuration before you add images into the gallery.
See: Global Configuration (Parameters - component in menu item)
"When using Multiple Add, you can choose the category you want the folder to belong to, then just tick the folder, and add it. If you are adding a lot of photos at the same time, remember it's YOUR server that is creating the thumbnails. So trying to add several folders to the same category at the same time is likely to crash. When I try and multiple add a folder with 200 photos in it I sometimes get a server error. Just hit the Back button on your browser and then click Save again. Make sure that the category you are adding to is still selected first though. You should find that thumbnail creation continues from where it stopped." - Jonathan
You can use multiple adding instead of creating category and adding one image into the category:
Go to: Components » Phoca Gallery » Images » and click on [Multiple Add] button. Now you are in Phoca Gallery [Multiple Add]. You can select the images or folder you want to add to the gallery here. If you select a folder and this folder contains subfolders and images, Phoca Gallery creates Category from selected folder, Subcategory from included subfolder and adds all the included images into this Category (Subcategory).
"This is the quickest way of adding images"