There seems to be no way to place buttons (drawings, images) within cells in a way that would allow them to be linked to Apps Script functions.
This being said, there are some things that you can indeed do:
You can place images within cells using IMAGE(URL), but they cannot be linked to Apps Script functions.
You can place images within cells and link them to URLs using:
=HYPERLINK("http://example.com"; IMAGE("http://example.com/myimage.png"; 1))
You can create drawings as described in the answer of @Eduardo and they can be linked to Apps Script functions, but they will be stand-alone items that float freely "above" the spreadsheet and cannot be positioned in cells. They cannot be copied from cell to cell and they do not have a row or col position that the script function could read.