Vector vs. Bitmap Images
When vector images are layered there are no background issues to worry about, but when a vector image is imported into a bitmap-based paint program or converted to a bitmap format the image is rasterized--destroying its vector qualities. For this reason, it's important to always use an illustration program when editing vector images, and a paint program when editing bitmapped images.
Transparency and Image Formats
The most common web graphics formats, GIF and JPEG, are bitmapped graphics. This means they will always have a background. "What?", you're probably saying... "surely you've seen images on the web with transparent backgrounds!" You're absolutely correct; this is possible due to the GIF89a format. GIF89a images have the ability to designate a single color in the image as transparent.PNG images also support transparency, but unlike GIF, PNG allows you to define more than a single color as transparent. Another advantage of PNG is that it supports millions of colors, whereas GIF is limited to only 256 colors. The problem with PNG is that not all browsers support it, and those that do are inconsistent in their support of it.
If you're planning to export an image as a transparent GIF or PNG, you must make sure that the program you are going to be importing to will accept that format. In general, you should only use GIF or PNG for transparent images you plan to use on the screen. For printed output, you should learn some of the other methods for creating transparency.
Continued: Masking and Clipping Paths
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