1. Display an image with a hardcoded path
The body of a Forena report is an XHTML file, which you can edit using the WYSIWYG report editor (ie the layout tab). So if you just add an image tag with a path pointing to your image, you can display such image anywhere in your report.
2. Transform the hardcoded image path into a Skin configuration option
Using Report Skins, you can use the technique of Defining skin configuration options. This allows for defining arbitrary variables that can be referenced in reports using the skin data context.
For example, assume you define a variable in the skin like so:
imagepath = path/to/image
Anywhere in your report, you can then refer to the token
3. Create a separate data block
If it makes sense for this specific question, you might want to consider just creating a separate (additional) data block related to the image to be displayed. That way you just retrieve the relevant data (row) from your
file_managed table (as in your question), and surround it with whatever HTML markup you'd want.
4. Available FORENA hooks for adding reporting extensions
Yet another option might be to use some of the available Forena hooks. Here are some of them that may help you:
Disclosure: I'm a Forena co-maintainer