TestRail can take any file format as an attachment, not just pictures. As such attachments are currently best access directly through TestRail. The only way to easily share information to users who do not have TestRail access is through emailing a scheduled report. These are intended to be small and provide an overview of testing progress. It is not intended as a means to provide highly detailed information as this could be problematic when dealing with emails. Especially once you start including images.
Alternatively it is possible to use the print function, save the print as a PDF or desired format, then share with your client. To do this, open the test run, select the print icon near the top right. On the pop-up window, look to the top left select “Details” from the dropdown. This will display any attachments that are viewable in a browser, such as images. From there select Print and save the output as a PDF. Windows 10 has this functionality builtin, for other OSes consider installing a free PDF printer like PDF Creator.