Thanks.  Very helpful explanation.  I've always used .jpg almost
exclusively and never noticed any degradation when editing.

Guess I'll have to re-learn everything I thought I knew ;)

Never did understand the need for 3, 5, 8, 10 or larger megapixel cameras.

I take all my snapshots at about 1/2 megapixel jpg and then crop and
further resize everything down to about 1/4th the original size, and I
can't tell any difference in image quality, even with a jeweler's loop. 
I've sometimes printed an original and a resized smaller version at Costco
and asked people to tell me which is better, and I've never found anyone
who could tell any difference.

People send these 3-megapixel (and bigger) images to me all the time and
they are really slow to load. So, I've always used imagemagick 'convert'
to bulk resize everything to about 1-20th the original size and they all
look the same to me.

On a recent vacation, I took more than 1,000 snapshots and by resizing
them, they all fit on a single CD with lots of room to spare. I also
upload our travel pix to a web page for our family to view online and by
reducing the image size, all the images load and display very quickly and
beautifully online.  With 3+meg image files it would take 20 times more
bandwidth and 20 times longer to load and display.

So, I just don't understand the benefit of keeping snapshots in gigantic
image file sizes.


-------
> TL;DR,
>   If you just want to have an image you can view and you want a smaller
> file size, then use JPEG and don't edit it.
>   If you want to edit the image or it's very small and speed of display is
> important, use PNG.
>
> The two file formats are quite different:
> PNG is *lossless* which means that you can edit, adjust, etc... the file
> without losing any image data.  It stores all of the data in compressed
> form, so it's larger, but everything from the original image is still
> present.
> JPEG is *lossy* it actually discards around 90% of the image data, so you
> can't edit a JPEG without losing some of the image quality; by the third
> or fourth edit a JPEG gets pretty bad.  It also uses some fairly complex
> math to store and reconstruct the image, so it's much more computationally
> intensive to view a JPEG compared to a PNG.
> The system (generally) uses PNG for thumbnails because (for small images)
> PNG is generally faster to create and faster to load due to less
> computation needed to compress/decompress data versus reconstructing an
> image from mathematical models.



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