Hi @SamAbbott,
The avatar image that appears when your emails are delivered to various inboxes is controlled and set by the inbox in question. For example, if your customers are receiving your emails in a Gmail inbox - that image would be controlled by a Google+ profile associated with your sending email address. At this time, many inboxes are moving over to being controlled by BIMI: https://www.brandindicators.org/, a process which must be set up by your business IT team. A few notable inboxes where images are now controlled by BIMI are AOL, Verizon and Yahoo inboxes.
Unfortunately, Klaviyo does not have any control over the image that is displayed within your customer's inboxes. Additionally, the above information is ever-changing as industry standards are adjusted according to the latest technology improvements.
With that said, if anyone else on Community has found another solution, we would love to learn about that as well!
~Chloe
Thanks Chloe for your response, it is much appreciated