In "A Photograph," what has not changed over the years? Does this suggest something to you?

The thing that has not changed over the years in "A Photograph" by Shirley Toulson is the sea. The poet writes,

. . . the sea, which appears to have changed less, washed their terribly transient feet.

The girls pictured in the photograph grew older; one became the speaker's mother, who took out the photo periodically and laughed with her daughter over the swim fashions shown in the photo. Fashions change; people grow older and die. The mention of "their terribly transient feet" symbolizes the way humans age. Their bodies change day-by-day and year-by-year until they eventually stop functioning and the person passes away.
The fact that the sea doesn't change provides a stark contrast to the ephemeral nature of life. The world has been here a long time, and it will continue a long time after each one of us who is now alive is gone. This can make the reader feel insignificant, or it can inspire the reader to live life to the fullest and to leave a legacy for those to come.
In the poem, the speaker's mother left a legacy of warm companionship that her daughter still remembers fondly. She remembers her mother's laugh and the way that her mother shared her own past with her. Those memories endure and come back to the speaker like waves of the sea come back to the shore over and over again. Forming strong relationships and sharing intimate moments are important ways that each of us can leave a link to the next generation even after we're gone.