How do I remove an image from a gallery?
Step 1

Go to the edit screen in your WordPress administration area for your gallery
Step 2

Click on the “Add Media” link
Step 3

Select the “Uploaded to this page” from the drop down menu to see only images associated with this gallery
Step 4

Find the image you want, click on the thumbnail, and find the “Delete permanently” link in red.
All set!