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Upload your image to an image hosting service. Copy the image URL. Add the image. Adjust the size. Save the signature.

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If you want to include an image in your signature, the image will need to be available online so that it can be linked to. You cannot upload an image directly from your computer to Gmail to use in the signature. You can upload your image to a variety of different services, including Photobucket, Blogger, Google Sites, Google+, or any other image hosting service. Once the image has been uploaded, you will need to copy the image's URL, or address. Depending on what image hosting site you are using, you may be given the URL when the image is uploaded. If not, you can right-click on your image and click "Copy image URL". The image URL should end with the image's file type, such as ".jpg" or ".png". Click the "Insert Image" button above the signature text field and new window will open. Paste the image URL into the field in the new window. If you paste the correct URL, you should see a preview of the image appear below the field. If no preview appears, then you likely did not copy the correct URL. If you are using a large image, then it will likely take up too much space in your signature. After adding the image, click it in the signature text field to open the size options. At the bottom of the image, you can choose "Small", "Medium", "Large", and "Original Size". Pick a size that allows you to see the image without taking up too much space in the signature. Since the image is being linked to and not actually included in the email, you don't have to worry about uploading it every time you send an email. Once you are happy with the way the signature looks, save your changes by clicking the Save Changes button at the bottom of the Settings menu. Your emails will now include your new signature at the bottom.