Image downloads are slow

[davidfoerster]

…for multiple reasons:

1. always loading a fixed amount of images independently of how many are needed,
2. sequential downloads,
3. always choosing the the image variant with the biggest pixel area, independently of file size.

[davidfoerster]

It would help to implement #11 to speed up downloads.

[davidfoerster]

#11 was closed and now at least reasons 1 and 2 are eliminated.

[davidfoerster]

Downloaded images are cached persistently on disk since 5e639a2. The default cache size is 50 MiB.