In "Death by Landscape," Atwood starts out by describing the paintings in Lois' apartment, noting that they were not peaceful to Lois. Rather, they filled her with "a wordless unease" as if there is something or someone looking back out. The reader is left with the expectation that perhaps the reason for this foreboding will be answered. In the first five paragraphs, all we are given is a lonely widow surrounded by a plethora of landscape paintings, all giving her a wordless, uneasy stare in return. This opening establishes a potential mystery to be answered.
The rest of the story does just this, recounting Lucy's disappearance. The mystery of the haunting landscape paintings is answered. I interpret the last line with a bit of optimism that Lois at least partially concludes that these landscapes which once haunted Lois now seem more like a windows which stretch beyond the dimensions of space and time. She notes that they are in fact, not landscapes but, foregrounds which retract endlessly away, paralleling the idea that Lucy is "entirely alive" just in spirit or memory.
Lois' actual experience with Lucy when they were kids recedes as time goes on; as does the foreground into the picture. As Lois gets older and her childhood recedes further into the past, Lucy recedes further into the foreground of the pictures; endlessly moving away but always there as if she's hiding always just out of sight. It's also endearing having the image of Lucy hiding under the canoe in the picture as if she's playing hide and seek, another nod to the notion of secrecy and nostalgia of childhood.