Hi @Matt Smith,
Welcome to the Community!
I believe this post here answers your question: https://community.atlassian.com/t5/Jira-Service-Management/Format-and-size-of-logo-for-JIRA-Service-Desk-customer-portal/qaq-p/154747
In that you should be able to select any or all of an image that isn't larger than 2000px x 2000px.
I hope that helps!
-Jimmy
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