SIGnatures
Somewhere in your email programme buried in the preferences/options is a section called Signatures.
A signature (or SIG) is a standard bit of text added to the end of your email. If you set up a sig then every new email for that account will have the sig automatically added to the bottom of the message. If you have more than one email account each account can have its own sig set up.
If you have a personal email then you can define any sig you like, from the sublime to the ridiculous.
For personal accounts a sig can be a bit of fun. For business accounts they are a must.

If you have a business then it is a good idea to have a standard sig for all your emails (sales@..., info@..., contact@... etc.). Though you will have to configure the sig for each account you can COPY and PASTE the information in from one account to another.

Images should be kept small as they take up a lot more “size” than plain text, and no one wants an email that takes 5 minutes to download!
Also it is a good idea to save them as GIF, JPG or PNG. If your logo is fairly simple with defined blocks of colour (e.g. a cartoon) then a GIF is probably the best format. More detailed artwork (e.g. photographs or complex colour blocks) will look best as a JPG or PNG.
The difference between JPG and PNG files is that PNG supports transparency. A JPG image will always have a background (usually white by default) but a PNG will permit an “invisible” background. JPG files are slightly smaller than PNGs so are generally the preferred choice.

Usually during the build of a site I will have a copy of that site’s logo and if you want me to create a logo for a sig then just ask.
Other typical sigs include web addresses, titles of some sort (e.g. Chairman of the Icelandic Bat Protection Society) or just some pithy comment about life, the universe and everything.
What do you want to say about yourself?
My favourite personal sig is a quote from Henry Thoreau...
“If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
"Let him step to the music which he hears, however measured or far away.”
[This is a simple piece of plain text in Courier font.]
All my karate related emails have this sig.

[This is a small JPG image with a white background].
Anything to do with my Vampilore web site has the sig below.

Everything your ever wanted to know about
VAMPIRELLA - Daughter of Drakulon
and a load of stuff you didn't know existed!
www.vampilore.co.uk
[This is a combination of a JPG image and some plain text].