Step 1.
Create a new field (See Creating Fields for help)
For this guide we can just call it 'Main Category'
Set field type to 'Drop Down List'
Add these options: Nature,Abstract,Architecture,Portrait,Macro,Closeup
Set Shorthand name to something (for example): Cat01
Set Smart theme name (Theme Group displayname) to something, for example: Main Category
Set 'index this field' to 'yes'