从以下的帖子中可以得出一个结论：
Google完全忽略网页中非链接图片的ALT文字，不知道是不是真的如此。至少得到肯定的是链接图片的ALT文字对Google有效，应该在其中加入描述被链接页面（注意不是本页面）的关键词 。
-Q
what happens when we use the same keyword within all ALT tags, after or before the actual image description. My question is: would Google pick this up as spam?
-A
First of all, the only alt attribute the search engines pay attention to are the ones on linked images. The alt for a non-linked image is ignored.
Now, if you're giving all of them the same alt attribute (it's technically not a tag), what you're doing is describing every page you're pointing to the same way, so you're kind of sending the wrong message. If every linked image's alt is "pet supplies" then no page is going to stand out from the crowd as being more about pet supplies than any other, so it's a self-defeating activity.
What you should be doing is describing the page you're pointing to with the linked image, using that page's keywords when it makes sense to do so. Remember that the alt attribute isn't just for search engines (nothing is, actually). It's supposed to be for users who can't see the image.