Is this a published report you accessed via web browser? I feel like I need a little more context.
The most common root cause to any error that states Reason: image not found is that the published workbook is missing external files. We can resolve this by re-publishing the workbook with the checkbox titled "Include External Files" checked. This will bring any images which are needed for the workbook if they have been embedded in a Dashboard.
Let me know if this works for you.