Placeholders are for text only, images can't be given as source. It looks like you want a background image in a text input/text area field for which you will have to use combination of cell styling and js.
Mention a cell style with a background image for the property. On click or focus, you can trigger a js which will remove this background image.
FYI, if you are trying for something like a search icon inside a input, check this out