Polygons are usually closed loops with lon,lat coordinates of all vertices, with one coordinate pair per record. Depending on the extent of the image, if you plot a polygon based on the 4 corners via gmt plot then we will compute intermediate points along the connecting great circles. However, you can can control that aspect via the -A option. For this stuff it is probably simplest to make an ascii table. If your images are rotated and in a projected coordinate system then things are a bit more complicated but gmt grdinfo -Ib can output the enclosing polygon in whatever coordinates the grid is in and you can inverse project those to get an oblique long, lat polygon.