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I am looking for a web app in which I can upload my images and convert them to web-friendly versions such as GIF/PNG? Something similar to when you upload photos to Facebook and they are automatically converted into GIFs.

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I don't use Facebook but do they really convert photos into gif? They'd look awful then. And gif is not really smaller than png with a limited color palette, it's not recommended for usage now. –  neo Jul 5 '10 at 14:39
    
@Neo, no; Facebook stores photos as JPGs. @J Angwenyi, JPGs are just as web friendly as GIF or PNG. –  Nate Lawrence Dec 22 '10 at 3:06

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up vote 5 down vote accepted

For photos please stick to JPEG, it's the best format for those out there. Graphics should be in PNG format. There are many converters available, here are some of the best:

If you have graphics in PNG format you can smush them afterwards which saves a few extra kilobytes.

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+1 for smushit! a great service utility. –  scunliffe Jul 5 '10 at 20:39

Zamzar will convert just about anything, including images

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Neo is right. Photos are far too complex to be stored in GIF or PNG with any smaller filesize than the JPG version of a photo.

PNG's main strength is saving a perfect pixel-for-pixel copy of an image and compressing it as small as possible, given the constraint of preserving each pixel's hue, saturation, lightness, and opacity exactly as they were in the original image. Their ability to save 256 different shades of transparency makes them ideal for saving non-rectangular images and superior to GIF (which can only save one shade of transparency). Photos do not contain enough repeated colors or patterns to be losslessly compressed to a small size. Since a PNG version of a photo is all about preserving each and every pixel with its original value and there are virtually no repeated colors in a photo, a PNG version of a photo will rarely be much smaller than a fully uncompressed format such as BMP. By contrast, a graphic such as the famous presidential campaign poster of President Obama, which contains only four colours, is highly compressible and a perfect candidate to be saved in PNG with a very small filesize.

As Neo also mentions, GIFs are not a good candidate for storing photographic images because the total number of different colors able to be stored in a single image is very low in comparison to other image formats. A GIF can only store 256 total unique colors for each frame whereas almost any photo will contain a list of colors with many more than 256 unique colors referenced. When a photo is converted to GIF, the 256 most common colors will be preserved and all colors outside that range will be changed to the closest one of these 256, however this will lead to posterization.

I mentioned the famous poster of President Obama from his election campaign before which is an artistic and intentional use of extreme posterization where the entire image has been reduced to only four colours, however when converting a random photo to GIF the result will not generally be pleasant.

JPGs use a completely different manner of compression known as lossy compression. This means that they are not at all concerned with preserving each pixel with the precise value as the original, but rather the same overall impression. JPGs are inherently lossy - literally unable to be saved at any more than 99 percent quality of the original image, however most programs which save JPGs as output (such as image editors) opt to save with a much more aggressive compression ratio such as 80 percent original quality or less. If there is a JPG image which you have repeatedly opened and saved in an image editor, this can rapidly degrade the quality of the image as each time the image is saved another 20 percent of the previous version is permanently lost (assuming that the program is set to save JPGs at 80% quality).

JPGs can be saved at any quality level you choose (south of 99 percent), however as you get into lower quality levels, the JPEG artifacts will grow increasingly unacceptable to human eyes.

A central high-range (75% - 85% compression quality) JPG copy of a photo will appear acceptable and will be much smaller in filesize than the PNG version of the same photo (although the PNG will be perfect quality).

Takeaways:

  1. JPG is an excellent format for photographic data being transported over the web.

  2. JPGs should not be repeatedly edited on one's own computer. If you will be repeatedly editing an image, it is better to save it in an uncompressed or losslessly compressed format such as TIFF, BMP, PNG, or Photoshop's PSD and only publish to JPG when editing is complete and the master copy is saved. Note: if preserving the original photo's metadata, such as camera model, exact date and time taken, etc., then keep an untouched copy of the original JPGs (assuming your camera is not saving images in a RAW format).

  3. If transparency matters to you, use PNG or JPEGXR (the next version of JPEG with the combined range of abilities of PNG and JPEG).

  4. Always archive the original photos. In ten years not even the original will have enough detail to satisfy you, however it will be a far better consolation than something which was permanently degraded for the internet speeds of a decade ago.

  5. Consider publishing your images in a multi-resolution image format if their primary use is to be viewed online. An example of this is the DZI format used by Seadragon. You can use http://zoom.it to convert any online image to a DZI and get an embed code for any site which allows you to post HTML code. Unfortunately there are not many tools for saving or viewing DZIs offline, though such tools are certainly possible for programmers to relatively easily write.

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Good summary! thanks for these notes –  Julius A Dec 22 '10 at 12:45

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