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What I think you're asking is whether having a decent-length ALT tag going to do you some harm? The ALT tag is designed to be a** descriptive alternative to the image** and therefore** should** describe the image. You might want to make this short, or it might take a sentence or two. Make this useful to people who browse with images off and use a keyword/phrase if appropriate. There should not be any disadvantage of a good descriptive ALT tag. Just be as concise as you can be without losing a quality description. If the description has value elsewhere on the page then do that too, but you will want to put an ALT tag in any way, which must still be descriptive but not a duplication of something you've already said on the page. Hope this helps.