You have selected free tutorial of the AutoDesk for the Adobe Certified Associate (ACA) :
Adobe Certified Associate in Graphic Design & Illustration Using Adobe Illustrator CS6 / CC
Topics : Identifying Design Elements Used When Preparing Graphics : Demonstrate knowledge of graphic resolution, graphic size, and graphic file formats for web, video, and print.
Adobe Help:-
For building websites you should always go with JPG *unless* your image is animated or has a transparent background. Then, use GIF. But, JPG is standard, supported by 99.9999999999% of Web browsers, low in filesize, handles colors better than GIF, and loads relatively fast. The primary web file formats are gif (pronounced "jiff"), jpeg ("jay-peg"), and, to a much lesser extent, png ("ping") files. All three common web graphic formats are so-called bitmap graphics, made up of a checkerboard grid of thousands of tiny colored square picture elements, or pixels. Bitmap files are the familiar types of files produced by cell phone and digital cameras, and are easily created, edited, resized, and optimized for web use with such widely available tools as Adobe’s Photoshop or Elements, Corel’s Paint Shop Pro and Painter, and other photo editing programs. PNG images for use on a website has obviously never built a site before. They aren't supported by all browsers and your visitors will be
disappointed when problems arise.
For efficient delivery over the Internet, virtually all web graphics are compressed to keep file sizes as small as possible. Most web sites use both gif and jpeg images. Choosing between these file types is largely a matter of assessing:
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The nature of the image (is the image a "photographic" collection of smooth tonal transitions or a diagrammatic image with hard edges and lines?)
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The effect of various kinds of file compression on image quality
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The efficiency of a compression technique in producing the smallest file size that looks good
GIF Graphics
The CompuServe Information Service popularized the Graphic Interchange Format (gif) in the 1980s as an efficient means to transmit images across data networks. In the early 1990s the original designers of the World Wide Web adopted gif for its efficiency and widespread familiarity. Many images on the web are in gif format, and virtually all web browsers that support graphics can display gif files. gif files incorporate a "lossless" compression scheme to keep file sizes at a minimum without compromising quality. However, gif files are 8-bit graphics and thus can only accommodate 256 colors. The format supports up to 8 bits per pixel for each image, allowing a single image to reference its own palette of up to 256 different colors chosen from the 24-bit RGB color space. It also supports animations and allows a separate palette of up to 256 colors for each frame. These palette limitations make the GIF format less suitable for reproducing color photographs and other images with continuous
color, but it is well-suited for simpler images such as graphics or logos with solid areas of color.
JPEG graphics
The other graphic file format commonly used on the web to minimize graphics file sizes is the Joint Photographic Experts Group (jpeg) compression scheme. Unlike gif graphics, jpeg images are full-color images that dedicate at least 24 bits of memory to each pixel, resulting in images that can incorporate 16.8 million colors.
