Hasta La Motion!

First of all, I did some graphics in photoshop (some sort of flowers) and saved it as a PNG with alpha.
I need to put this on a wall and to reveal the flowers slowly from left to right.
I tried to do it with the paint node by animating a stroke (bigger than the flowers) that affects the alpha channel of the PNG but it doesn’t work since it affects the original alpha, revealing a black background behind the flowers because the stroke is bigger than the graphic itself.
Is there a way to do what I want? I’m sure there is, but I can’t seem to find it!

I also have a problem with the tracker node.
The footage is a side view of a women walking down the street and the camera is following her. I want to track some points on the wall behind her to put my graphics on. Everytime I start the tracking, the point gets lost after a couple frames.
I tried to track a point on the women and it works perfectly, but it doesn’t work on the wall.
I did put my tracking points on bricks corner and stuff like that, but they keep sliding.
I also tried to colorcorrect the footage a little to give it more contrast, thinking it would help tracking but it still doesn’t work.
The footage comes from a Sony HDR-XR550V camera and I desinterlaced the footage.

I hope I made it clear enough. Sorry if it’s not, my english isn’t perfect.

Thanks you so much for your help!