//I know IplImage is outdated
IplImage* im = cvLoadImage("1.jpg", 1);
//....
bgr2yuv(im->imageData, dst, im->width, im->height);

void bgr2yuv(unsigned char *src, unsigned char *dest, int w, int h)
{
  AVFrame *yuvIm = avcodec_alloc_frame();
  AVFrame *rgbIm = avcodec_alloc_frame();
  avpicture_fill(rgbIm, src, PIX_FMT_BGR24, w, h);
  avpicture_fill(yuvIm, dest, PIX_FMT_YUV420P, w, h);
  av_register_all();

  struct SwsContext * imgCtx = sws_getCachedContext(imgCtx,
                                         w, h,(::PixelFormat)PIX_FMT_BGR24,
                                         w, h,(::PixelFormat)PIX_FMT_YUV420P,
                                         SWS_BICUBIC, NULL, NULL, NULL);

  sws_scale(imgCtx, rgbIm->data, rgbIm->linesize,0, h, yuvIm->data, yuvIm->linesize);
  av_free(yuvIm);
  av_free(rgbIm);
}