Open Google Photos on your phone or go to photos.google.com on a browser
Sign in to the Google account that contains the photos
Go to the Photos tab
Select the pictures you want to delete
To select multiple photos, click or tap and hold one photo, then choose the rest
Click or tap the Trash/Delete icon
Confirm the deletion
Open the Trash/Bin folder
Empty the Trash/Bin to permanently delete the photos
On mobile, go to Library or Collections, then Trash, then Empty trash
On desktop, go to Trash, then Empty trash
If you want to delete all photos at once, use the web version and select multiple photos in batches
Repeat the selection and deletion process until all pictures are removed