I have two banners I am using in my site. They are currently
rectangles about 900 px wide. I would like to somehow crop or do
something to get the corners rounded but not feather/faded/or
blurred. I would like to just put a thin border around the photo so
it looks clean but have the corner rounded.
Make a rounded-corner rectangle the size you want with the
corner radius you like.
Duplicate it and put the copy on a new layer.
Make the first one with
white fill and no stroke. Select your banner and the
Modify > Mask > Group as Mask.
Think of masks like stencils...white shows everything, black
shows nothing, intermediate colors show by luminance.
On the copy or your rounded rectangle, set the fill to none
and the stroke to the border thickness and color you'd like. Since
it's a copy of the shape you used to mask with, the border will
match up exactly.