It is very easy to remove unwanted images from the PhotoBulk's Gallery. Just click the Trash icon button in the bottom left corner of the main window.

Alternatively, you can delete a single image by right-clicking on the thumbnail preview at the bottom of the window and choosing Delete from Gallery from the contextual menu. You can delete all images by choosing Delete All from Gallery.

Tip: To delete all images from the Gallery, press and hold the Option key and click the Trash icon button.

You can also choose File → Delete Selected Image from Gallery (⌘⌫) or File → Delete All Images from Gallery (⇧⌘⌫) from the menu bar.