No... but if you have some SEO company who has done work for you asking you to place their logo and link on your site, I'd tell them hell no. It will do you absolutely no good, and will only help them. I detest this practice as much as web designers who put a link to their site in the footer of sites that their clients pay them to design. They got paid for doing the work... why give them the extra benefit of a link unless it was negotiated in advance and some type of compensation was made to the client like reducing the price of the design work in return.
no way, otherwise all the SEO sites would have been penalized as well. SEO sites are just like any other sites. you can link to them or have a logo
There is nothing wrong with placing a link to the seo company on the website, why not give thanks to the company that has helped push you to the top. And of course it helps the seo company, how else do you think most top seo companies have climbed to the top of the search engines for some of their main keywords.
no I don't think so... in face the alt text of the logo would even help your sites rankings a tiny bit
One of our products we have worked very hard to get in the top 10 of Google organics, we read a lot and tried a lot of best practices. Do a quick search for link juice on Google and you will see how link building and site quality are affected by who you link to and who links back.