Sylpheed ebuilds since 0.7.7 (at least) do not enable gdk-pixbuf or imlib unless USE="gnone" is set. I have gdk-pixbuf, but I do not have USE="gnome", so sylpheed gets built without imaging support. I hardcoded --enable=gdg-pixbuf in an ebuild, and now things are back to normal.
Here's the thing, gdk is actually closer tied to GTK than GNOME. The problem is that many GNOME apps take for granted that gdk is a GNOME thing. We tried to implement an ebuild for gdk-pixbuf which takes advantage of the gnome USE flag, but that caused more problems than it solved. So for now, what you did was the best way to do it. The good news, however, is that GTK2 will come with gdk, and this will all go away once gtk apps start getting ported to gtk2..
