Animation bumps upwards on the second frame

So, when my character's walking animation enters the second frame, it bumps upwards a little. The frames are almost the same sized, other is 2 pixels larger in x scale. Why does this happen and how can I fix it?

As you can see, in the second frame, the leg is a bit above the ground and the camera reacts to that as a bump. (And yes, that is a remake of the protagonist of Contra, fully remade and recoloured by me, and is temporary character with the exact same x and y scale :P)

<img src="http://i.imgur.com/TX71mp0.png" border="0" />

(Second frame is on the left, first one is on the right)

It's likely that your origin points are different in each frame. To ensure they're all in the same place (not always desirable, but it probably is in this case), open the animation editor for your sprite, click the origin button on the left, right-click on the origin in the panel that opens up and click "apply to whole animation".

It did not fix it, but I did notice that the origin is not in the middle of the frame, does that affect, and if yes, how can I pinpoint the center of the frame?

In your animation editing window, go to your first frame and center the origin point (or hit 5 on the number pad) then right click the origin point in the image points list and select apply to entire animation.

It still does the little bump on the second frame.

if your animation frames aren't the same size, make sure that the distance between the origin and the bottom is always the same (e.g.:I put the origin at the lowest point in the middle for my character in the case of a platforming game), also, place it not with the mouse, but with the keyboard, to be sure that the coordinate is truly the same

That should prevent the bumping

Also, verify your collision polygons

Yeah, it was the collision polygons. I had set them a bit off on the second frame. Thanks.