Once I was trying to resize a jpeg image and made some Internet search. Believe or not, I couldn't find clear answers to my question, but it's very easy to do.

The code below will reduce width and height of a chosen .jpg image.

Go to "File / New / Console Application" and paste this code. Set the SizePct (a const on the code below, but can be a variable on your program) to fit your needs. If you want a new image with 30% of the original width and height set this to 30.

All I do is load the JPEG on a TJPEGImage, create a bitmap and .StretchDraw the JPEG on the bitmap. Then I copy the bitmap to a TJPEGImage using the .Assign method, and, finally, save it.

Enjoy!

// ----- Code Starts Here -----

program Project1;

{$APPTYPE CONSOLE}

uses
  Classes, Windows, SysUtils, Dialogs, JPEG, Graphics;

const
        SizePct : integer = 50; { The new image will have 50% of the original }

var
        OpenDlg : TOpenDialog;
        SaveDlg : TSaveDialog;
        oJPG    : TJPEGImage;
        oBmp    : TBitmap;


begin
        OpenDlg := TOpenDialog.Create(nil);
        SaveDlg := TSaveDialog.Create(nil);

        if (OpenDlg.Execute) then
        begin
                try
                  begin
                        oJPG := TJPEGImage.Create;
                        oJPG.LoadFromFile(OpenDlg.FileName);
                  end
                except
                        MessageBox(
                                0,
                                PChar('Error while trying to open ' +
                                OpenDlg.FileName +
                                '.'),
                                PChar('Error'),
                                MB_OK or MB_ICONERROR
                        );
                        exit;
                end;

                oBmp := TBitmap.Create;
                oBmp.Width := Round(oJPG.Width * SizePct / 100);
                oBmp.Height := Round(oJPG.Height * SizePct / 100);
                oBmp.Canvas.StretchDraw(
                        Rect(0, 0, oBmp.Width - 1, oBmp.Height - 1),
                        oJPG
                );

                oJPG.Assign(oBmp);
                oJPG.Compress;

                if (SaveDlg.Execute) then
                begin
                        oJPG.SaveToFile(SaveDlg.FileName);
                end;

                oBmp.Free;
                oJPG.Free;
        end;

        OpenDlg.Free;
        SaveDlg.Free;

end.