Unfortunately you're not on the right track.
Never mind the fact that your string is totally messed up (double quotes opened but not closed, field name enclosed in braces on one side and square bracket on the other, etc.).
As I said before, you'll need VBA and events for that. Don't put anything in default value property of your new field, leave it empty. Open your form in design mode, place unbound Image object on it. In property window of form look at Event tab. There will be event called "On Current". Click three little dots next to it, select "Event procedure" if asked, that will open VBA editor with something like:
Code:
Private Sub Form_Current()
End Sub
Between those 2 lines goes your code. You want Image to be filled with your photo. So code will be something like:
Code:
Private Sub Form_Current()
Me.Image1.Picture = "C:\Users\marlee\Documents\Clarence River Museum\Photos\" & Me.Field3 & Me.Field2 & ".jpg"
End Sub
You really need to get some grasp of VBA first, I'd suggest watching some introductory videos or courses. And of course ask here if you can't understand something.