Your eMail signature is stored locally in the eMail program (email client) that you use. If you use a number of different eMail clients; Outlook, WebOutlook, your Phone - you will need to update the signature in each one.
PLEASE: follow the Style Guide - do not attempt to add images to your email .sig.
For Outlook 2016 - on your School laptop. Use this article - https://support.office.com/en-us/article/Create-and-add-a-signature-to-messages-8ee5d4f4-68fd-464a-a1c1-0e1c80bb27f2
For OutlookWeb - from the Office365 Portal page - use this article - https://support.office.com/en-us/article/create-and-add-an-email-signature-in-outlook-on-the-web-5ff9dcfd-d3f1-447b-b2e9-39f91b074ea3#bkmk_beta