The leaflet, promoting Cardiff North candidate John Dixon, shows him talking to a woman in nurse’s uniform.
But the identity of the ‘nurse’ has been revealed as a researcher for the Liberal Democrat AM Mick Bates.
The Lib Dems said the picture was meant to be “illustrative”. Opponents have described it as “appalling” and an attempt to mislead the public.
Jonathan Morgan, the Conservative AM for Cardiff North, said: “They should apologise to my constituents and to the nurses working in Cardiff who will be astonished at this behaviour.
“It is appalling that the Lib Dems in Cardiff North would seek to deliberately mislead my constituents by using a picture of their candidate talking to a nurse, when I know she is not a nurse and has never been a nurse.”
But a spokesperson for the Liberal Democrats defended the image: “The photo in the leaflet was designed to be illustrative to help us highlight the party’s commitment to improving the NHS and we didn’t intend to cause offence to any health professional.
“Our candidate in Cardiff North has a strong record of improving health and social care in the community.”
But the picture has also come under attack from others.
A Welsh Labour spokesperson added: “Its one thing pretending your leader could be Prime Minister, quite another having your staff pretend to work for the NHS.
“Every picture tells a story, and this picture says you can’t trust a Lib Dem.”