Salix OS

1. Start Thunar
2. Edit > Configure custon actions...

3. For use with a single file (%f)
3.1. Edit Action (Basic)

Name: Open image with feh
Command: feh %f

3.2. Edit Action (Appearance Conditions)

File Pattern: *.gif;*.jpg;*.jpeg;*.png;*.svg
Appears if selection contains: Image Files

4.1. For use with selected file(s) (%N)
4.1.1. Edit Action (Basic)

Name: Open with feh
Command: feh %N

4.1.2. Edit Action (Appearance Conditions)

File Pattern: *.gif;*.jpg;*.jpeg;*.png;*.svg
Appears if selection contains: Image Files

4.2. For use with selected file(s) and folder(s)
4.2.1. Edit Action (Basic)

Name: Open with feh
Command: feh %N

4.2.2. Edit Action (Appearance Conditions)

File Pattern: *
Appears if selection contains: Directories, Image Files

5. For use with a folder
5.1. Edit Action (Basic)

Name: Open directory with feh
Command: feh %f

5.2. Edit Action (Appearance Conditions)

File Pattern: *
Appears if selection contains: Directories

6. For use with folders (--recursive)
6.1. Edit Action (Basic)

Name: Open directory with feh
Command: feh -r %f

6.2. Edit Action (Appearance Conditions)

File Pattern: *
Appears if selection contains: Directories

Slideshow with feh: (by julian67)

7. For use with current folder
7.1.Edit Action (Basic)

Name: Slideshow with feh
Command: feh -p -F -Z -D 3 %F

7.2. Edit Action (Appearance Conditions)

File Pattern: *
Appears if selection contains: Directories, Text Files, Audio Files, Video Files, Image Files, Other Files

8. For use with folders (--recursive)
8.1.Edit Action (Basic)

Name: Slideshow with feh
Command: feh -r -p -F -Z -D 3 %F

8.2. Edit Action (Appearance Conditions)

File Pattern: *
Appears if selection contains: Directories, Text Files, Audio Files, Video Files, Image Files, Other Files

Thunar Custom actions: feh + Slideshow