Zoom in on Still Image with a mask

It is not totally clear if you mean the same region of the original image, or the same area of the frame.

In any case, it is not at all difficult to achieve. Suppose you have an image like this and want the mask to correspond to the marked rectangle:

Situation A: when you zoom in, you want the rectangle to grow too; so you mask the same area of the image all the time.

Solution: just add the mask to the clip

Situation B: when you zoom in, the size of the mask remains the same, and any part of the image outside the fixed size rectangle is masked out.

Solution. Turn the image into a compound clip. Add the mask to the compound, but zoom the image.

I can tell you in more detail how I did the example shown in the GIF file in my previous post.

At the bottom is the image of Babar, king of the elephants - call it clip B (for background)

The image with names of characters of the series is above it - call it clip F (for foreground)

It may look like this:

Now I want that only a rectangular area around the first three names is shown and Rataxes is masked out.

I could apply a shape mask to F and it will look fine:

Now if I scale F (e.g. to see the text more clearly), then I get this:

The whole rectangle grows and partially covers Babar.

That is "Situation A" on my previous post.

Hang on for "Situation B"...

Alternatively, we may still want to zoom in the text, but keep the size of the rectangle the same - for example, to emphasize the word "Celeste".

So we now we go back to the step before adding the mask:

We select the F clip in the timeline:

and press Option-G to create a compound clip. We may rename it "F Compound" for clarity:

We now add the mask to "F Compound":

It looks just like the previous version, but now we scale the original F clip.

To do this we need to double-click F-Compound.

A slight inconvenient of doing this is that when we drill into the compound we will not be able to see the clip B.

In the compound clip timeline:

we select the F clip and scale it up, in the Inspector or the Viewer.

Then click the < button above the timeline to go back to the project.

Notice how the contents of the rectangle were scaled up, but the rectangle is still the same size - now showing the word Celeste in a larger size. Go back and adjust if necessary (for instance, here we can still see the edge of the word "Babar" in the corner):