Sending multimedia content and documents within chat conversations in Microsoft Teams
From my findings, there are other discussions of similar behavior. You can see a related Microsoft Q&A discussion here: When I view (in Teams) an image sent to me in Teams, the resolution is good. When I download it, th…
As a first step, I recommend checking whether the same behavior occurs in both Teams desktop and Teams on the web. Also, if possible, test with different image files and different chats. This can help determine whether the problem is specific to certain attachments or occurs generally.
As a workaround, since you need the original resolution for your design work, you can ask the sender to upload the original image to OneDrive and share the OneDrive link with you instead of sending the image directly in Teams. You can then download the original file directly from OneDrive.
Since this is a user-to-user support forum, I don't have access to your organization's Teams environment or the backend needed to investigate the issue directly. If the issue persists and you would like it investigated further, I recommend contacting your organization's IT administrator and asking them to raise a support ticket with Microsoft on your behalf.
If you’re unsure who your IT administrator is, see How do I find my Microsoft 365 admin? - Microsoft Support