How can I use function 'measure' inside 'grid' to align two images?

[SillyFreak]

It looks like this is your problem:

Why is the value I receive from context always content?

When working with context, you need to put everything that depends on the contextual information into the context block/expression itself. The explicit context expression is a trade-off: You get to know where in the document you are (accessible via here()) and which set rules are active there (accessible via things like text.lang). In return, the context value itself becomes opaque. You cannot peek into it, so everything that depends on the contextual information must happen within it. Let’s lo…

In short, you need to pull context out of the image function:

  context image("image2.jpg", height: measure(image("image1.jpg")).height)