Your image must be publicly hosted externally. This can be on your company domain, or some other file hosting service. It is recommended to scale the image to the correct size BEFORE you host it.
Navigate to Tools > User Options OR Click on your name on under the search bar.
Scroll down to Email Signature section and Click 'Insert > Insert image'
Source: Enter the URL where the image is hosted.
Image description (Optional): Enter the text you want displayed if/when the image doesn't load.
Dimensions: Enter what size you want the image to display. 'Constrain proportions' will lock the image ratio to match it's original (so it doesn't look skewed). The image's original dimensions will populate the fields by default. If you hosted the correct size don't modify this.
Click OK and Click Save
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