It is important to note that the size of the picture you are inserting into your signature must be changed before inserting. You cannot change the size of a picture after it is in your signature.
First open your inbox online.
Click the gear in the upper right hand corner of the screen.
In the search box, type the word "signature" without quotations. This will populate 1 result. Click the result that says "Email Signature"
A new box will open up, as pictured below, where you can type your signature. Clicking the circled icon will allow you to browse your computer for the image you wish to insert. After changing your signature, click "Ok" above the words "Email Signature" to save and close the window.