Check if you have hidden picture files in the folders with names like folder.jpg and/or albumartsmall.jpg.
It could be that they still show the old image.
Fi you want to get rid of these files once in a while, open a command shell window in the top folder and enter the following commands:
attrib -s -h /s *.jpg
del /s folder.jpg
del /s albumartsmall.jpg
This deletes all of these files.
You may now, if your smartphone relies on these files to show covers at all, export the cover files from the music files with the help of Mp3tag:
Load all the files into Mp3tag.
Execute an action (quick) of the type "Export cover".
Enter as filename: folder
This creates a file folder.jpg in each album folder.