Given $2n$ points $x_1, x_2 \ldots x_{2n}$ and a distance $d_{i,j}$ defined between them, how can I best find the set $P$ of mutually exclusive pairs $(i,j)$ such that the sum of their distances

$$ \sum_{(i,j) \in P} d_{i,j} $$

is minimised? The definition of $d_{i,j}$ is open and the function could be convex. The motivation for this problem is practical. How can I pair of 30 pictures say into most similar pairs?

I apologise in advance for the choice of tags on this post. I have been out of maths proper for a long time.