1) From the back end "administrator" interface, go to Components -> Ignite Gallery.
2) Click "Manage Images" in the row for the category corresponding to the gallery you'd like to edit.
(In the left side "Category" should be highlighted in blue indicating you are in Category list view.)
3) Drag images into the GRAY box labelled "Drag files here" OR click "Add Files", then browse to the images you want to upload.
4) Once you have selected all the images to be uploaded, click Start Upload.
Once the image shows in the list and is "Published" (should show green check box rather than red circle), the image should show in the gallery.
5) Choose the Edit link in the image's Description column to edit the image details. Documentation on editing the image details is here.