ah, yes, good, that way we're sure we iterate on a fixed list, a snapshot before the changes.

so with this, and a check on the mime type, we get something like :

-----
local outputfile = PANDOC_STATE.output_file:gsub("%.md", "")

local function prefixName(s)
  return s:gsub("([^/]*)$", outputfile .. "_%1")
end

function Image(img)
  img.src = prefixName(img.src)
  return img
end

function Pandoc(doc)
  for _,f in ipairs(pandoc.mediabag.list()) do
    local fp = f.path
    local mt, contents = pandoc.mediabag.lookup(fp)
    if mt:match("^[^/]*") == "image" then
      local fpnew = prefixName(fp)
      pandoc.mediabag.insert(fpnew, mt, contents)
      pandoc.mediabag.delete(fp)
    end
  end
end
-----