The insert picture menu option or script step may be used to select a file to insert into a container as a picture, but there is no facility in place for designating the specific folder.
If that is a critical requirement, you may need to acquire a plug in that adds that capability.
On the other hand, you could use import records | Folder to import all the picture files in that folder into a table and then the user could review the images in a list view of that table and click to select one for linking it to a specific record in your database.
i was assuming it could be done as filemaker lets you click on a container and be given the option of inserting picture!
Just noticed I think my question was wriiten wrong
(revised question below)
I want to have a script which will do the following from a button
allow the user to select a picture which will then send it to a container.
That doesn't change my answer.
Yes, you can open a dialog where you can select a picture to insert into the container field.
No, you cannot specify that the dialog always open to a specific folder.
Frustrating, but true.
You can work around that limitation by one of the methods I mentioned earlier.