REPRODUCIBILITY (% or how often): 100%
BUILD ID = OS VERSION (Settings > About product): 4.3.0.12
HARDWARE (XA2, X10, X10 II, …): Xperia X
UI LANGUAGE: DE
REGRESSION: (compared to previous public release: Yes, No, ?): Yes
Gallery app shows all (music) album cover (cover.jpg) images.
newly flashed OS
pls checkout https://together.jolla.com/question/5935/how-to-blacklist-directories-from-being-indexed-by-tracker/ (asked december 2013 
).
But there are alternative gallery apps, like imageorganizer or SailPhoto ( 
)
Yep.
.nomedia does the trick (note to myself .trackerignore was a few devices earlier ).
But there is also a patch ‘Tracker path settings’. I do not know if it still works but it can be applied.
works like a charm. Thx you, guys. 
Doesn’t seem to have an effect here (XA2). Tried touch ~/Music/.nomedia as well as putting it in specific sub directories next to a jpg.
(It does work to exclude certain sub directories in ~/Pictures, but not for ~/Music.)