Thumbnails for JPEG images are not generated

When I open the resource browser, I can see thumbnails for .gif and .png files, but no thumbnails for .jpg or .jpeg files. By lookiing at phpinfo(), I know my GD library setting includes GIF, PNG, WBMP and XBM support, but JPEG is not mentioned (don’t know if it’s default). I just use the GD library (2.0.28) came with PHP bundle, and have no idea how to configure that. Can someone shed some lights about it?

Thanks,

Ryan