Some image file formats, like PNG or GIF, use only lossless compression, while others like TIFF and MNG may use either lossless or lossy methods.
JPG (= JPEG) and GIF, destroy some of the image data during the file-saving process. For that reason, you should save in JPEG or GIF only after you're completely done editing, and only if you want to use the picture for some purpose that requires one of these formats.
Image Size - Keep your images as small as possible (resolution, colour depth, file type, etc.). Remember that there may still be some users without a broadband connection.
Warning! - Make a copy of your original image. Work with copies. Never work with the original file (unless you can go to the source and get another copy).
Shrinking Digital Pictures
Resize your images
Constrain proportions
Width: 400 pixels to 600 pixels (px)
or Height: 300 pixels to 400 pixels (px)
Change either Width or Height, not both.
72 ppi (pixels per inch) is adequate for the Web.
Always work on a copy of your original picture file. NEVER work with the original.
Some people still use a monitor resolution setting of 800 x 600, but some users will view your photos on screens using a lower or higher resolution.
For pictures that you plan to attach to an e-mail message, limit the picture height to 300 pixels and the picture width to 400 pixels.
For Web pages, the same pixel dimensions work fine as long as your page has only one image. If you have multiple photos on a page, you should keep your pictures even smaller. Each photo you put on the page adds to the page download time, and you don't want visitors to your site to have to wait several minutes to display the page.
HTML
use the width attribute with the src tag to specify a percentage of the width of the user's screen rather than a specific number of pixels.
Spacer.gif is a pixel shim. It is a 1 pixel * 1 pixel transparent GIF. If you would like a copy of it, just right-click on it and Save Image As... to your local disk.
Transparency isn't the only advanced feature that PNG boasts over GIF. PNG images can also contain a gamma correction value, which is meant to represent the relative brightness of the display on which the image was created. In theory, the browser can use this value to display the image at the exact same brightness on another display.
That's all well and good, except that other image formats and solid colours specified with CSS do not contain this gamma correction value, so when you adjust the display of a PNG based on its gamma information, it no longer matches the other colors on the site.
For this reason, most browsers (including current versions of Firefox, Safari, and Opera) ignore the gamma correction value in PNG images when displaying them on the Web. But Internet Explorer does not, and that's why my PNG images stood out in that browser.
The solution to this problem is to produce PNG images with no gamma correction information in them, so that Internet Explorer will not attempt to correct their display. Unfortunately, Photoshop's "Save for Web" feature doesn't give you that option, so you need to use a separate tool to strip out the gamma correction information that it writes.
A great tool for doing this in Windows is TweakPNG. It's open source and a free download. Simply drag a PNG to the program window and delete the "gAMA" chunk from the list before saving the file. While you're at it, you might as well delete the "tEXt" chunk as well, to save a few extra bytes of file size by removing Adobe's stamp on the file.
PNG was designed for transferring images on the Internet, not professional graphics, and so does not support other color spaces (such as CMYK).
PNG itself does not support animation.
PNG offers a variety of transparency options.
PNG-8 - 8-bit - similar to GIF - more advanced compression than GIF, lossless compression
PNG-24 - 24-bit - similar to JPEG, lossless compression, may be larger than JPEG
offer DivX files in a highly compressed version of the MPEG-4 format used by corporate outfits like Microsoft and Real Networks. Because of the powerful DivX compression technique, which is called a codec, these files can be relatively small and yet contain high-resolution movies that play in full digital grandeur on a PC or Mac monitor.
MyDVD uses a simple drag-and-drop interface that allows users to combine video, audio, text and pictures into "video postcards" with up to one hour of high-quality video on a low-cost CD recordable disc. US$50