set the value of a field to the value of a primary key?

Hello, I have a problem reggarding a commenting script that i am forming so that users can leave comments for particuler images.

I have a submission form that takes in the comment, then i need to assign an art_id in the comments table.

Artwork can have one or many comments.

this one to many relationship means i have the foregin key 'Art_id' in the comments table.

I need to know the SQL that would be needed to form something like...

select art_id from artwork then take the value and set it to art_id in the comments table.

can any one help with this?
many thanks

http://art.uk-graffiti.co.uk