The image is from a chat group called Army of Mujahideen and appears to be designed by a user called Dr Almani.
Another image shows a blood soaked hand gripping a knife, Photoshopped onto a poster for Paris Christmas markets, which has the Eiffel Tower in the background.
The emergence of the images comes after German police arrested six alleged Isis plotters amid fears they were planning an attack on a Christmas market on the anniversary of last year's atrocity in Berlin.
The Syrian men were arrested during raids involving 500 officers in the towns of Kassel, Hannover, Essen and Leipzig.
Prosecutors say they were plotting a co-ordinated attack using "weapons and explosives".
There are reports the target may have been a Christmas market in the north western town of Essen on December 19, but this has not been confirmed by authorities.
On the same date last year, Tunisian Isis fanatic Anis Amri hijacked a truck to murder 12 revellers at the packed Breitscheidplatz Christmas market.
The Foreign Office has issued advice for travellers planning attend the seasonal events across Europe this year.
It says: "There may be increased security in place over the Christmas and New Year period, including at Christmas markets and other major events that might attract large crowds. You should be vigilant and follow the advice of local authorities."