Allegro collision detection

bool Player::collide(int x, int y, BITMAP* object)
{
    for (int px = 0; px < 20; px++)
    {
        for (int py = 0; py < 20; px++)
        {
            if (getpixel(image,px+x,py=y)!=bitmap_mask_color(image))
            {
                if (getpixel(object,xPos+px+x,yPos+py+y)!=bitmap_mask_color(object))
                {
                    return true;
                    break;
                }
            }
        }
    }
    return false;
}

xPos and yPos speak for themselves
x and y are the potential changes to the xPos and yPos, if this method returns false, they will be made.
image is a 20x20px player image.
object is my arena made from a tileset, pieced together by a vector of strings, each character in the vector representing a tile.

However, my player resolutely refuses to fall when gravity tells him to, or the program crashes. Either he's colliding with something (He shouldn't be, he's surrounded by masked squares) or some crazy voodoo is eating my datas.

any ideas?

I hope xPos and yPos are relative to the other object.
Also, why are you colliding a thing with an image?
Also, why aren't you testing bounding boxes first?
Also, why are you including velocity in collision detection? If either of the velocities are over 40, then there is a chance that they will pass right through each other.

I bet the rest of this dreadful thing isn't any better. 2/10 on a logarithmic scale at best.