Look at the picture!A desk is it

on the picture 表示在图片表面上的 物体,
eg. If We clean the dust on the picture, it may seem more beautiful.
in the picture 表示在图片里面的物体.
eg. How many birds do you see in the picture?
I can see five birds in the picture.But there are so many fingerpirnts(指纹) on the picture.

希望帮到你,谢谢采纳!
再问： 应该用什么？
再答： 根据前文“瞧这张图片”，应该是图面里的一张桌子，所以是“in” 谢谢，麻烦你采纳