Within the TrepStar Cart settings section in your TrepStar account, you can add custom HTML in the head, body, and footer sections of the TrepStar purchase page (buy now buttons / cart page).
Many people choose to include logo images, css, js, and any other valid HTML.
However, you must use https for any outside references other than simple a href links to other web pages.
Otherwise your customers will think that the purchase page is not secure! They probably will not purchase from that page and certainly your sales will be lower than they could be.
If you've added http:// instead of https:// to any of the TrepStar custom HTML sections, you'll see a warning that brought you to this page.
If you see that warning, you should modify your links. If you do not, your customer will see warnings like this (example from FireFox):
Here is more information about this issue.
Change your references to https:// If you don't have https on your server, create an account with amazon s3, put your files there, and link to those public https urls to customize your cart.
Look at this video explaining how TrepStar publishers may publish their large DVD iso files to Amazon S3 or to the TrepStar FTP page, but you can use the FREE amazon S3 service to upload and reference your image and other files via secure server. Amazon S3 files are capable of being served securely (https). Once you have your files on S3, make them public, copy the public URL (which will be https) and use that in your cart settings on the TrepStar account.