First, you must use an email app that is capable of composing email in HTML, aka Rich Text. Not all apps can. Check to see if your email client has a Rich Text option. I think K-9 and Maildroid do, and my favorite, AquaMail does. I use it to send email with logo signatures, like you're trying to do.
Second, when you try to paste in your signature and logo, what you are actually doing is pasting in a bit of HTML code. Most HTML (Rich Text) editors don't allow that. They want to generate the code themselves, by you typing in the text, selecting the image, etc.
Once you know that you can send HTML email, you need to either get a copy of the logo image so you can compose the signature yourself, or maybe a company IT person can help you with it.
Android since v1.0. Linux since 2001