Desktop Photo Gets Changed

Ever since I installed macOS Mojave, I can't get my choice of desktop photo to stay put. If I shut down my computer, when I restart it my personal desktop photo has disappeared and in its place I have one of the generic desert sand dune photos in its place. I have to again go through the process of changing the desktop photo. I don't want to see the sand dunes -- I want to see the photo of my dog against the green, grassy expanse of our backyard! Before anyone suggests that I wouldn't have this problem if I never shut down my computer, please know that the power company shuts it down for me on an all too frequent basis.