There are two options on the web page how you can add items, which you can both find in the Import menu: Import Snapshots and Add Items.
Import Snapshots is for importing items from data you have entered inworld, after clicking the "Snapshot" button on the 12o'clock position on your HUD.
Add Items lets you add items from images, textures, or Marketplace URLs.
In both options you can drag in multiple images at once. The Import Snapshots window will automatically associate the images with the data you entered, while in the Add Items window you'll still have to enter the necessary details to create items, but it will use the file name of the images as title.
So in both ways you could create items pretty quickly if you have the data prepared correctly.