Is it true that I can not insert an image or attachment to templates unless they exist at a url? Follow

Yes, this is partially true.

Templates can be used to send the same email to multiple people, so every sent email will cost more space. Consider if there is a 2mb file attached to an email, and if you send that email to 200 people then it will consume 400 MB of data for your instance. The best way is you can use google drive, dropbox, one drive, etc ( or any other storage provider ) to upload a file once and use it again and again. It will save space and will also give you an idea about who opened that link or not.

If you would like to insert images you can use our Email Media Manager option.