1. GIF Format

   It  is the most  appropriate export format for graphics with large areas of solid colors. Digital artwork using a narrow palette of  colors (up to 256)    will deliver crisp and a small file size when  exported as a GIF.

   The GIF format is also a good choice to make certain colors transparent in the exported image:  index transparency. That comes handy when you need  transparent edges around your image. You will be then able to blend that exported image with any web page area no matter what background color is used.

   Your only choice is a GIF format to create  animated graphics such as banners (animated gif) unless you use a Flash movie:   JPEG and  PNG do no supports a such animations. You would have to either keep the number of animation frames to a minimum and reduce the dimensions (width and height) to lower the file size of your “animated gif”.

2. JPEG Format

   This format is more appropriate for images with lot of details and photos requiring millions of colors. The JPEG format is also the best option for  computer graphics with gradients of colors or  artworks using shadows, bevel, feather and other digital effects.

   Images  exported in a JPEG format are usually crisp when exported with a compression between 60 and 80%.  That  gives you enough wiggle room to find a balance between quality and file size.

3. PNG Format

   The PNG format comes in three flavors: PNG8, PNG24 and PNG32. This format lays somewhere between the GIF and JPEG formats. PNG is a lossless compression, uses virtually unlimited palette of colors and offers both index and alpha transparency.
   Let’s explore what each subset of this format has to offer.

   • The PNG8 is  quite similar to the gif format in terms of quality and size of the exported file. It offers index transparency as well as alpha transparency
   • PNG24 is better suited when your graphic is a mixture of lots solid colors and gradients. In a nutshell, the png24 format lays between the gif and jpeg formats.  It produces crisper images with a smaller file size than the GIF format.  However the PNG24 does not offer index or alpha transparency.
   • The PNG32 option is currently not available in Photoshop. You will have to use Adobe Fireworks to take advantage of  the full alpha transparency offered by the PNG32 format.
   • The only drawback of  PNG images is they are not well supported by Internet Explorer 6: you will have to resort to a CSS hack using Microsoft’s filters.

• Although the GIF format offers a palette of 256 colors, my personal experience taught me to use the GIF format only when no more than 32 colors are needed to keep the exported image sharp. For images requiring more colors, you are better off using the other file formats.
• If the image you are dealing with needs more colors than the GIF format can handle and no transparency is required then JPEG is your best bet.
• There is no real gain in choosing the PNG format at this point unless you are using the PNG32 to take advantage of alpha transparency you cannot get with GIF or JPEG format.

In the end there are really no hard rules in choosing the right file format to export your image in. As you just learned, the best suited file format depends on the current context: the graphic you are working on.

1. Always start with picture dimensions (w, h) greater than your target size
2. Set the image resolution between 72 and 96 pixels/inch
3. Crop the image to only keep the area needed to convey your message
4. Resize the cropped image to reduce the dimensions to the minimum while maintaining the proportion between width and height
5. Choose the appropriate export file format: PNG, JPEG or GIF
6. Select the right compression ratio/color range

To put it in simple terms, you need to find the right balance between the quality and file size.
If you must do some corrections such as removing blemished or creating smoother edges, do it on the original image (before resizing it) .

I have seen websites using a 500kb (kilo bytes) picture they shrunk by just setting the desired with and height in the HTML image tag (<img src=”path-to-image” with=”300″ height=”250″ />).
That trick does not reduce the file size of that image and in most case the picture displayed is less sharp than what it should be. You have not only a degradation in the picture quality but a waste of bandwidth and web hosting storage. No wonder that page took so00 long to download.

To give you an idea, a raw picture resized to of 300 x 250 pixels and well optimized should be in the range 5 to 10 kb depending on how rich in colors that image is.

I have also come across web pages where some picture are “bloated” to dimensions greater than the actual width and height. Of course a few pixels adjusted never hurt and are barely noticeable: it solves a presentation issue and saves you some time.

However a change of an image’s dimension (width or height) from 150 pixels to 200 pixels would catch the eye and obviously degraded the visual appeal of that image.

If you follow the 6 key points listed above, you will get a crisp image with a small file size.