Do you mean as an image description tag? If so then the effect will be minimal. If it does not match the logo and you opt into enhanced image search it may damage your serp for the keyword. If you stuff it with a gazillion words the very minor influence would be watered down. Stick to 2 or 3 words that describe the logo correctly.
anything that is included in the website for the sole purpose of manipulating the SERPS and that has no benefit to the user is considered black hat and can potentially get you banned
Do you mean alt text or title text? Apart from those two I don't know what other means you can possibly hide text within a logo.
Don't use that its a black hat seo technique if you are using this then it will very harmful for your site. Google can read easily because you can hide on your internet explorer but what about the source of this page you can't hide from there its my opinion according to me its bad technique. Cheers Aryan http://onlineunsecuredbusinessloan.blogspot.com/ http://badcreditunsecuredpersonalloansuk.blogspot.com/
That's a blackhat technique don't hide your keywords these can be seen as keywords spamming. Just use alt tags on your image and insert your keywords related to the image.