(Brittany Watson/Cronkite News) SAN LUIS – Sculpted calaveras – skulls – are perched alongside a painting of Frida Kahlo, which hangs next to several rocks used by artists as improvised canvases. They’re all part of an exhibit that recently opened to the public at the San Luis Library, located on the border with Mexico about 25 miles south of Yuma.Organizers said the art helps to unite two cities that, although separated by walls and fences, share a common history and culture.“This exhibit is about providing opportunities for artists in our region . . .