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The steps to place an image in the signature in office 365 outlook:
1. Upload the logo to a public access url.
2. In Outlook 2010, compose a new email and make sure the Format Text is HTML. Click Insert>Picture, paste the URL of the logo in the File name tab and click Insert. Please send the email to the Office 365 email address.
3. In Outlook Web App, double click the email, right click the logo and click Copy. Click Options>See All Options>Settings. In the E-Mail Signature edit box, press Ctrl+V to paste the logo. Mark the Automatically include my signature on messages I send option. Make sure HTML is select behind Compose message in this format. Click Save.
4. Try to send an email to check the result.