I use three dos batch files.
File 1 is called xGen.bat, and is called by the second one.
REM change the next line to match your installation
set IVIEW="C:\Program Files\IrfanView64\i_view64.exe"
REM you can toy with these values
set FILTERS=/resample /aspectratio /sharpen=5 /jpgq=85 /contrast=20
REM really only needed once.
mkdir Thumbs
REM change jpg to PNG if that's what your source files are
%IVIEW% %1.jpg /resize_short=%2 %filters% /convert=".\Thumbs\%1_%2w.jpg"
File 2 is called xGenAllSizes.bat, and contains this:
REM you can remove lines or add new ones
REM the value is the length of the SHORTEST side in the converted file.
call xGen %1 60
call xGen %1 180
call xGen %1 500
call xGen %1 1600
Finally, file 3 is the one you set up with all your cover files in. This is called something like Resize.bat, and it's the one you actually run.
Don't put spaces in the filename, or you'll need to make some changes to all the files.
Don't include the extension.
Contents of file 3:
call xGenAllSizes Hal1
call xGenAllSizes Hal2
call xGenAllSizes Hal3
To use, put all three batch files into a folder with your covers, and add 1 line for each cover file to the Rename.bat.
Double-click Rename.bat and you'll end up with a Thumbs folder full of various useful sizes.