That probably doesn’t work unless you power-cycle the picture frame after changing the photos.

I had this with some offline Samsung picture frame and a Transcend WiFi SD card. The SD card runs a small Linux and can be unlocked to add own scripts. I had a script that would rsync files from my storage to the SD. However, while the new files were written to the SD just fine, the picture frame never re-read the list of files from the SD. And after power-cycling, my specific model needed to be turned on manually again. So, that wasn’t a satisfactory solution.