Sparkle’s recompression is actually quite smart. I added a 3.5mb image to a sparkle page and output it and found the image file size had been reduced to about 360K and absolutely no perceivable loss of quality. I then added a pre-compressed image - same size dimensionally but about 360k in size, and guess what, sparkle didn’t recompress it by any significant degree (about 10k at most) - and there were no artefacts in the output image. However, the big difference for me is that the sparkle file size for the larger image was marginally over 3.5 mb, but the sparkle file size for the pre-compressed image was a much more acceptable 360k. Now, it may not make a lot of difference to app performance if there are only one or two images, but where you may have a photographer’s site with maybe 50 or a 100 images, pre-compression does make sense.
I’ve recently completed a site for a photographer that contained 82 images. Had they all been placed at their original file sizes, the Sparkle file would have been 582mb. But pre-compressing the images, the file size came in at 27.8mb, and yet there was no appreciable recompression in the output files.
So, if you want smaller project files, pre-compression is a good way to go - Sparkle is smart enough to know whether an image has already been optimised and does very little in the way of extra compression. Of course, if you don’t mind working with large project files, then Sparkle will do an equally good job at outputting the final web-ready images.