MusicBee like some other players will not read album art in the .webp format, found increasingly on the web and notably in the excellent last.fm photo collections. Here is a batch file that will allow you to select one or more image files in the webp (or others!) format and convert them to jpg from the right-click Windows context "Send to" menu, before adding to your music files via MusicBee's tag editor. The original file is kept, the converted one will have the same name with the *.jpg extension.
This method depends on having the freeware image viewer/editor IrfanView installed. The WEBP developers have their own command line tool but it will not convert to jpg, hence the workaround. Copy the bat file below into a text file like "BatchConvert to JPG", changing the path of Irfanview to match yours and "i_view32.exe" to "i_view64.exe" if you have the 64-bit version
. Then, save the result as a *.bat file in the Windows "SendTo" folder. The latter is normally at C:\Users\(username)\AppData\Roaming\Microsoft\Windows\SendTo.
Then, you can select one or more webp files (using ctrl-click, etc), right-click and choose "Send to-->BatchConvert to JPG.bat", which will bypass the IrfanView GUI and quickly convert the files into *.jpg, keeping the originals.
echo The file(s) will be converted to JPG
if "%~f1"=="" (
echo No more args given. Done.
exit /b 0
C:\Windows\System32\cmd.exe /c "%ProgramFiles%\IrfanView\i_view32.exe" %1 /convert=%~n1.jpg