Complete and utter nonsense with Overlay and Graphics

Question

I'm trying to simply overlay some text and a scalebar onto an image. I'm getting (what I perceive to be) completely ridiculous results while trying to do this.

Here's the code:

img = Import@"http://i.imgur.com/PVUXYz5.png"

Manipulate[
overlay = 
Graphics[{Text[Style["50 Magnets", White, 30], {250, 250}], {White,
   AbsoluteThickness[5], Line[{{i, 200}, {i + 50, 200}}]}}];, 
{i, 1, 450, 1}]

Dynamic@Overlay[{img, overlay}]

The only thing I'm Manipulateing, as far as I can see, is the x-values of the points where I'd like my Line to show up. When I increase i, I expect the Line just to move from left to right. Instead, the line shrinks and grows in size and translates around.

Not only that, but the Text moves around, too! I cannot for the life of me see why this is happening. I expect that I have a misunderstanding of how Graphics is working, but even breaking it up into two Graphics objects and combining them with Show results in the exact same behavior.

Does anybody know why this happens and what I could do to fix it? Ostensibly, adding two words and a line to an image (and mapping this operation to a list of images once I figure this out) shouldn't be difficult, but this may be the oddest behavior I've ever seen from Mathematica and I'm just totally clueless.

Answer 1

In addition to @kglr comment. I would recommend placing all in a single Manipulate and making use of Prolog for the img background. The Graphic is actually resetting its PlotRange as it is redrawn. This is why you need to set it as constant.

Manipulate[
 Graphics[{
   Text[Style["50 Magnets", White, 30], {250, 250}],
   {White, AbsoluteThickness[5], Line[{{i, 200}, {i + 50, 200}}]}},
  PlotRange -> Transpose@{{0, 0}, ImageDimensions@img},
  Prolog -> {Inset[img, Center, Center, ImageDimensions@img]}],
 {i, 1, 450, 1}]

Hope this helps.