To remove images from a Quickgallery, just edit the page with the gallery. Then click on the “Add Media” button above the editor, then it open a pop-up window. In this window choose “Uploaded to this page” in the dropdown field, then WordPress will show you only the images attached to this page, these are the images in the Quickgallery.
To delete one (or more) of the images, just click on the image itself, then on the right side appears a dialog, there click on the link “Delete permanently”.
After this you can close the pop up window, save the post and all is done.
Please note: If you use the quickgallery shortcode ([quickgallery id=”xxx”]), then please edit the page with the id in the shortcode, because the shortcode display in it the images uploaded / attached to this page.