Progressive?

Hi Adrian, 

I've added this to the next release.  If you're comfortable modifying PHP code you can try the following changes.  Be sure to create a backup file first.

1. Created a backup of /cmsb/lib/image_functions.php
2. Edit file and search for: save target image
3. Add the code in red:

  // resample image
  $quality = 4; // v2.60 Speed up resizing (was 5 previously)
  _fastimagecopyresampled($targetImage, $sourceImage, 0, 0, $dst_x,  $dst_y,  $targetWidth, $targetHeight, $sourceWidth, $sourceHeight, $imageType, $quality) || die("There was an error resizing the uploaded image!");

  // enable progressive JPEGs
  imageinterlace($targetImage, true);

  // save target image
  $savedFile = false;
  switch($imageType) {

Note that only newly created thumbnails and uploads will be interlaced.  But you can go into the field editor and click "recreate" for pre-existing thumbnails if you want to regenerate them.

Let me know if that works for you!