Are you referring to uploading images in a thread? If so there is an option to insert image as full size when you initially upload the image. Past that I don't think there's an option to make it so all images are full size for guest because its the users choice when they upload and attach it, either full size or thumbnail or leave it as an attachment. BUT you could work around the permissions by:
You can set in global permissions here:
Admin Panel-> Users-> User Group Permissions-> Unregistered / Unconfirmed -> View attachments to posts
If you wanted to restrict it to a certain node browse to: