Is it possible to create formatted email on the iphone 4s?
iOS 5 brought some limited rich-text formatting options to Mail. When you're composing an email, select the text you want to format, and tap the right arrow. You'll see a button labelled "BIU", tap that and you get options to bold, italicize or underline the text. You can also control the indentation level.


(Image from Macworld)

As you can see, it's fairly limited, and to my knowledge there's no way to insert a signature with any images. You could keep the logo you want in your iPhone's Pictures and copy/paste it as necessary, but that's obviously very labour intensive.

Your could look into alternative mail apps. There aren't a lot to choose from, but something like MarkdownMail might work well for you. It's just meant for composing HTML formatted emails, you'd still use the built-in Mail app for checking, etc.