TASImage image("$ROOTSYS/tutorials/image/rose512.jpg");
UInt_t yPixels = image.GetHeight();
UInt_t xPixels = image.GetWidth();
UInt_t *argb   = image.GetArgbArray();

TH2D* h = new TH2D("h","Rose histogram",xPixels,-1,1,yPixels,-1,1);

for (int row=0; row<xPixels; ++row) {
   for (int col=0; col<yPixels; ++col) {
      int index = col*xPixels+row;
      float grey = float(argb[index]&0xff)/256;
      h->SetBinContent(row+1,yPixels-col,grey);
   }
}

gStyle->SetPalette(kCubehelix);
h->Draw("colz");

gROOT->GetListOfCanvases()->Draw()