Idealy you'll want to have an xscale and yscale variable which you'll lerp to 1 in the step event. Have a scale_max and scale_min value, which should be somewhere between 0.7 and 1.3 When you jump set xscale to min and yscale to max, when you land set xscale to max and yscale to min. Add draw_sprite_ext to the draw event and use the scale variables in the image_xscale and image_yscale arguments.
Lastly ensure the origin is bottom middle of the sprite. If you do all that right, it should work fine.