Hi Sonja,
Yes, your picture is a true conch shell, probably the Caribbean "fighting
conch", Strombus pugilis.  This particular conch however is only about 3
inches in length, so it is probably not the one you are using as a
centerpiece.  Perhaps your shells are the big "queen conch" or "pink conch",
Strombus gigas?  Do you know where they came from?  However, your
description of a "salmon-colored" interior suggests that your shells may not
be true conch shells, but perhaps one of the "horse conchs", which belong to
a different family altogether.  Also, it is not true that all shells are
known as conchs, or even all large shells.  The name refers primarily to
members of one family, Strombidae, but a few shells in other families have
picked up the name (horse "conchs", crown "conchs", etc.)
Paul Monfils