Are you referring to JIRA server or Cloud?
If server, please which version and I would be happy to help have a look.
If cloud and the image is already attached, click on the image(upload file or image icon in the comment menu) as shown below:
Select the file you want to attach and it should be added.
If you are having problems, can you take a screenshot of what you see when you attempt to upload a file or a screenshot of your comment section.
If I misunderstood your problem, do help clarify this and I would try to answer.
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