Difference between .png and .jpg photos and how to convert from one to the other.

Joined
Apr 27, 2010
Messages
18,190
Location
Suburban Washington DC
I wan't to save the presale pictures of a car I bought at auction. Previously I could drag and drop them to the desktop, not now. After some trying, I could copy and paste them but only as .png photos but the problem with that is my photo viewer doesn't show them. It can only see .jpg. I can convert them in Paint but the size dropped from 400 KB to 70 KB. Is there a way to convert them to jpg without losing so much detail?
 
There is typically a setting when you go to save to adjust compression on jpg's. You want 0 compression.
 
Look @ GIMP for Windows; free, powerful

 
Use .JPG; any "quality" between 75 and 85 will be a great blend of small file size and hardly any perceptible loss of quality.

There are oodles of free online converters. This is my go-to:

 
You can use the free FastStone Photo resizer to batch convert from PNG to JPG. You can specify the quality as well.

I do already have FastStone but didn't know you can specify the quality as well. Thanks. Instead of going from 400 to 70 KB I was able to adjust it to the max at 240K. Good enough for just some archive photos. What is .png and why is it used vs. jpg?
 
What is .png and why is it used vs. jpg?
PNG is a lossless format. JPG is lossy. Kind of like FLAC vs MP3.




PNG is good for diagrams and text, to preserve sharpness and eliminate artifacts when zoomed in, but for high res photos on the web, it is usually too large to be practical.
 
Last edited:
.png's are also used if you need a transparent background, if you have an image of a logo you want to put on a website for instance you only want the parts to the logo to show, hot the whole image (square).

A good example would be the logo on this site, go up and left click on it and start dragging it off to one side and you will only see the logo move not the background.
 
Their compression systems are different, too (PNG uses LZW compression like GIF and TIFF. JPG uses some type of magic voodoo :) ). Aside from .PNG's ability to support transparency, it can net you a better file size *and* better image quality for images whose content is mostly large swaths of flat colour (like a logo) whereas .JPG/.JPEG (Joint Photographic Expert Group) works really well fudging details in a very complex image with all sorts of lines and colours and colour gradient (like a photo of the real world: Because of the complexities of real light, almost all of the pixels have different colour values so compression comes at the expense of accuracy and detail; but done in extremely clever ways that are based on how our brains perceive those details).

So that's always been my rule of thumb: PNG if 1) transparency is needed and/or 2) it's a generated graphic and/or 3) I need an absolute minimum of image degradation and I don't care too much about file size. JPG if 1) it's a photo and/or 2) I need the smallest possible file size with reasonable image quality. TIFF if I need something in CMYK colour space for print. (PNG and JPG are RGB colour and, for the most part, do not support CMYK. If you have to print a JPG or PNG then something - either the software, library in the OS or the printer itself - converts it to CMYK, which might get inaccurate.)
 
Last edited:
What I run in to a lot with Photographers (most of my customers are photographers) is they don't understand that the best the web can display is 72 dpi (resolution). You can put a 300 dpi image on a website and a the same image at 72 dip and you can't really tell the difference.

When I assemble websites for a customer I don't even tell them anymore that I drop the res to 72. I also do this in case someone copies/drags the image off the site it will print grainy.
 
I'd just get a photo viewer app capable of PNG. I usually prefer PNG because storage space went up so much in the last decade/longer, and PNG I can losslessly edit later. If 100% sure I'm not going to edit later, I save to JPG quality 94% or for web, mid-80's. Web diagrams, I save as PNG 256 color pallet or grayscale, have a photoshop plugin called Ulead Smartsaver Pro which makes that very easy to do and compare the output quality side by side with the original, as well as the file size while adjusting settings. I'm sure there are other apps that do this too but the plugin... meh I got used to using it years ago and still works so no incentive to try anything else.
 
You can actually just change/edit the file type extension from .png to .jpg (and vice versa) ... at least I can on my computers.
 
Look @ GIMP for Windows; free, powerful

I use Gimp in Ubuntu, it works great
 
Back
Top