Re-saving JPEG while keeping quality

Yes, there will be loss of quality, even at 100%. That said, is it likely to be all but unnoticeable immediately. If you have Photoshop you can easily confirm the loss of quality:

  • Load up the original and the resaved image as two layers in the same image.
  • Change the top one's display mode to Difference.
  • Select All and then Copy Merged (Ctrl+A and Ctrl+Shift+C by default).
  • Paste the result in as a new layer.
  • Using the Magic Wand tool, (Tolerance 0, Contiguous unchecked, Antialias unchecked), select an all-black pixel (RGB 0, 0, 0).

If all pixels are selected (which won't happen), the two images are identical. Otherwise the unselected areas have been modified.

If you resave the same JPEG a few times it will become much more noticeable.

Any sort of resizing that is not 1:1 aspect ratio will affect the quality, however, if you do not change the size or any properties, a 100% quality export / save should not affect quality.

However, I have never used the tool you are using. What I said is just general experience from using Photoshop and other photo editing software.

(Also, you can always use Imagemagick on Windows!)

Lastly a bit of general advise, Jpeg is not really a good format for anything professional, you should use a lossless format and then just use Jpeg for the distribution ... Although, for every day stuff, Jpeg is more than sufficient.