In order to prevent abuse some search engines will reject keywords if repeated too many times. It is probably best not to repeat them more than five times. Images are a great place to hide keywords in your document. When you insert an image in your document you have the option of adding text that will show if the image isn't displayed. In some web tools this is called an alternate.
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They can only be read by reading the source code. Word Tracker - Find out what the heavily searched keywords are.