How to delet photos?
Open Gallery > tap and hold a folder to start highlighting which folders/albums you want to delete > Once you highlight one, tap on the others you want to delete > After highlighting all folders/albums, tap the delete icon.
Like Carl P said, tap and hold an album, and then you can see "Select all" on the bottom, tap it and press "Delete" to remove all the files.
Also, you can try another free app - Apowersoft Phone Manager. This app also has the function to delete files. Open this app on your phone and go to "Photos", hit "Select" > "All" and then tap the delete button on the bottom left to remove all.