How do I make scaled images look better in Java/OpenGL?

Here are two pictures of the same image:

this is what it looks like when viewed in preview...

and this is how it looks in my game...

Obviously the character is scaled down in my game, however, when I scale down the sprite in preview or pixelmator, it is still smooth. Even when the image is at a 1:1 scale it still has the jagged edges.

This is my render method

public void render(Camera camera, List<Entity> entities) {
    shader.loadViewMatrix(Main.createViewMatrix(camera));

    //per textured model
    GL30.glBindVertexArray(((RenderComponent) entities.get(0).getComponent(EntityComponentType.RENDER)).texturedModel.getRawModel().getVaoID());
    GL20.glEnableVertexAttribArray(0);
    GL20.glEnableVertexAttribArray(1);

    for(Entity e : entities) {
        RenderComponent render = (RenderComponent) e.getComponent(EntityComponentType.RENDER);

        //load transformation matrix per entity
        shader.loadTransformationMatrix(Main.createTransformationMatrix(render.position, render.rx, render.ry, render.getScaleX(), render.getScaleY()));
        shader.loadSprite((AnimationComponentContainer) e.getComponent(EntityComponentType.ANIMATION));

        //activate texture for each entity
        GL13.glActiveTexture(GL13.GL_TEXTURE0);
        GL11.glBindTexture(GL11.GL_TEXTURE_2D, render.getTexturedModel().getTexture().getTextureID());

        //render entity
        GL11.glDrawElements(GL11.GL_TRIANGLES, render.getTexturedModel().getRawModel().getVertexCount(), GL11.GL_UNSIGNED_INT, 0);
    }

    //per textured model
    GL20.glDisableVertexAttribArray(0);
    GL20.glDisableVertexAttribArray(1);
    GL30.glBindVertexArray(0);
}

And in my fragment shader I sample the image

vec4 color = texture(sampler, pass_TextureCoords);

and remove transparency

if(color.a < 0.5) discard;
out_Color = vec4(totalDiffuse, 1) * color;

My question is, how do I prevent the jagged edges from occurring (they are most prominent around his pants)? Or how do I smooth them out? The image itself if 512x1024