This page basically lets a user choose some images so that he can process them.
Initially, the page will look like this:
I would like to somehow temporarily fill up the empty space while the visitor decides to upload his pics.
After the visitor selects his images, the page will look something like this:
What can I do to fill up that white space?
I would show light gray generic versions (like a "watermarked" look) of the grid for the blank page. This will imply that something is missing, and will encourage your user to take action and do something.
If the container will always be the same height, just set a height on it. Otherwise, perhaps you could set a min-height on it.