Pixels and Paths
I just don't get it. Shouldn't pictures be pictures? Why make a big deal out of this vector-raster stuff?
The two types of file formats are not generally compatible. Each has its strengths and weaknesses. Objects and lines created in Illustrator are typically made up of paths that may or may not be filled with color. These paths themselves may be colored or may be blank. The computer file that stores the image needs to know the relationship of each path to the others in the image and the colors used for each stroke and fill. When the file is opened and the image is viewed, the computer can make it any size required, as long as the relationships among the paths and their fills and strokes are maintained. The capability to resize images without degrading their quality is one of the things that can make vector formats preferable to raster formats in many cases.
Raster images, such as photographs taken with digital cameras or scanned into a computer, don't have paths and objects. You may see shapes and lines in the picture, but they weren't recorded as such in the image's file. Raster files record only a series of little squares (pixels), each a specific color. Neighboring pixels may be of the same color, different colors, or shades of the same color so close that you cannot see the difference. The picture is re-created by displaying those thousands or millions of little colored squares. Raster images are typically better than vector for displaying subtle shifts in colors and hues, such as those found in photographs.
I want to get my files really small for the Web, but it takes a long time. I save an image at one compression level and open it up to look at it. Then I have to go back and try a different compression level to get what I want. Isn't there a way to preview an image being saved as a JPEG?
Use the menu command File, Save for Web. This feature allows you to compare up to four versions of the same image, all at the same time. You pick the versions that best meet your needs for quality and file size, and click OK.
RGB for the Web
I created JPEG files of a lot of the images my company used in its latest brochure so that I could post them on the Web, but they don't show up. I exported them as JPEGs, so what's the problem?
For a brochure, eh? I'll bet those images were CMYK. Although that color mode is per-fectly acceptable for JPEG, it's not acceptable on the Web. Your images need to be converted to RGB before they can be seen by a Web browser. Throw away the CMYK JPEGs and go back to the originals. Use Save for Web rather than Export, and the color mode will automatically be converted for you.
What's that? Why does JPEG accept CMYK if you can't see it on the Web? Well, JPEG does more than just Web graphics. It's also one of the major file formats used to compress images for storage or archiving. And many of those images are CMYK.