Learn English – How to say “to start establishing a family”

I would go with (close to your last option):

We define an anchor as a textual element that marks the start or end of a data region, or as a feature within a region that distinguishes it from the rest of the page.

You can leave out distinctive because it is redundant since you also use distinguish which has the same connotation.
I would not leave out page since it is not already defined in this passage.

Five seconds to show the image

This means that you have five seconds to show the image. The problem here is that the implication is that the user is going to be the one showing the image.

Five seconds are left to show the image

Same problem here. This means that the user has 5 seconds left to show the image.

Five seconds are left until showing the image

This doesn't have the same problem as before, but I still don't like this one. It sounds awkward. You could make it sound better by saying "until the showing of the image," but that is too verbose anyway.

five seconds are left until the image to be shown

This is missing a verb. It should be

Five seconds are left until the image is to be shown

However, this is too verbose in my opinion.

For something like counting down time until a puzzle ends, I find this to be common enough and not be too verbose:

Five seconds remaining until image is shown