Tiled Gallery Block: potential iteration on the reordering feature

Image reordering was added in #14702. It works well, but can be a bit confusing at times when the layout does not really make it clear which image is next in the ordering sequence. Here is an example:

https://videopress.com/v/xdcgqFMU

I am not quite sure what could be done about it, but it can be confusing for some folks.

Start by reviewing the tiled gallery block reordering feature introduced in #14702 and the linked video example. Determine how the current layout makes the next image unclear, then define and validate a clearer ordering interaction for the block.

Written by the indexing model from the issue text.