#include <pngimage.hpp>

//...

TPicture *p=new TPicture;
HWND hwnd=GetDesktopWindow();
HDC hdc=GetWindowDC(hwnd);
TCanvas *C=new TCanvas;
C->Handle=hdc;
int w=Screen->Width;
int h=Screen->Height;
p->Bitmap->Width=w;
p->Bitmap->Height=h;
p->Bitmap->Canvas->CopyMode=cmSrcCopy;
p->Bitmap->Canvas->CopyRect(TRect(0,0,w,h),C,TRect(0,0,w,h));
ReleaseDC(hwnd,hdc);
delete C;
TPngImage *img = new TPngImage;
img->Assign(p->Bitmap);
img->SaveToFile("C:\\Users\\Администратор\\Desktop\\image.png");
delete p;