Delphi png、bmp、gif等图片格式转换成jpg

 unit Unit1;

 interface

 uses
   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
   Dialogs,  ExtCtrls, StdCtrls,jpeg;

 type
   TForm1 = class(TForm)
     btn3: TButton;
     img1: TImage;
     procedure btn3Click(Sender: TObject);
   private
     { Private declarations }
   public
     { Public declarations }

   end;

 var
   Form1: TForm1;

 //png,bmp等图片转jpg 函数
 function ConvertPICintoJPG(cPic: TPicture;  pWidth: Integer = ; pHeight: Integer = ): TJpegImage; stdcall;

 implementation

 {$R *.dfm}
 function ConvertPICintoJPG(cPic: TPicture; pWidth: Integer = ; pHeight: Integer = ): TJpegImage; stdcall;
 var
   tBMP: TBitmap;
 begin
   Result := TJpegImage.Create;

   if (pWidth > ) or (pHeight > ) then
   begin
     try
       tBMP := TBitmap.Create; //创建一个过渡性BMP图片,用于更改图片尺寸
       if pWidth <=  then pWidth := cPic.Width; //若pWidth为有效值则改变tBMP宽度,否则不变
       if pHeight <=  then pHeight := cPic.Height; //若pHeight为有效值则改变tBMP高度,否则不变
       tBMP.Width := pWidth;
       tBMP.Height := pHeight;
       tBMP.Canvas.StretchDraw(tBMP.Canvas.ClipRect, cPic.Graphic); //按照新尺寸重画图形
       Result.Assign(tBMP);
     finally
       tBMP.Free;
     end;
   end
   else Result.Assign(cPic);
 end;

 procedure TForm1.btn3Click(Sender: TObject);
 var
   myjpg: TJPEGImage;
   myimg: TImage;
 begin
   myimg := TImage.Create(self);
   myimg.Picture.LoadFromFile('D:\pro\plt_17313.png'); //加载图片
   myjpg := ConvertPICintoJPG(myimg.Picture,  myimg.Picture.Width, myimg.Picture.Height);   //调用转换函数
   myjpg.SaveToFile(ExtractFilePath(ParamStr()) + 'tmp.jpg');   //保存图片
 end;

 end.

使用jpg图片 需要引用 jpeg 单元