If I´m right....
resizing and compressing pic directly on the server will cause heavy duty and needs a lot of time and memory - usually the servers aren´t configured for that.
So the download and doing by hand is the best way. Anyway - You wouldn´t need each pic in the same size.
If You don´t have a copy of Your web-content (which You should have

) make it right now. Resize the fotos as You need and transfer it back.
Local You could use any album-program such as JAlbum to resize the entire folder within seconds and transfer it via ftp - this means plenty fotos in the time of 2 or 3 through the server.
The named programm allows You to keep the original, make a copy with desiered size (640x480) and jpg-quality (smoothing and compression) and creates a thumb in a choosable size - 1 setting - 1 click - plenty fotos.
Better do it like this.