Hey @morcholski,
This is happening in a flow triggered by Started Checkout, right? I’d recommend taking a look at the image source in the Started Checkout event that this person triggered. If copying and pasting that source from the event data into your browser takes you to the image, that means that Klaviyo is receiving a working image, and adding it to the HTML of the template.
If that’s the case, then this issue appears to be related to how Outlook renders emails, rather than how Klaviyo creates them and sends them out. Are there any other images in the email that are working, or have all of the images been removed, and replaced with the "Error! filename not specified" language? If some images are working, you might want to compare the image URLs of the images to see if there are any major differences in the URL structure of each one. It might also be useful to reach out to Outlook’s team and ask them for clarity on why this error message occurs, and how to avoid it in the future.
Let me know if I can help with anything else, or if you have any questions about the above!
-Byrne