can someone tell me how can i do a picure on a cake please?
Do you mean an edible image print? Drawing a picture maybe?
You could do a Frozen Buttercream Transfer. Theres an article on here that explains how. http://cakecentral.com/article12-How-To-Create-a-Frozen-Buttercream-Transfer.html
Or, you could use an edible image printer to print out a photo with edible ink, edible paper etc.
I go to a local grocery store and they would print an edible image for me from a picture. Check with your local bakeries or grocery stores if you do not have a printer for this.