If you're using plain JIRA issues to handle test sessions, then look for the "attachment" options under the issue, or drag them into the comments while editing.
If you're using some add-on designed for testing, then you'll need to read the docs for that, as we can't help you without knowing what it is.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.