Dimwit,
I have very little experience of MJ14, but I have lots of experience of MC14. MJ14 is based on the MC14 code. I have not seen a similar problem reported in the MC14 forum board.
MJ12 was based on MC12. You can find a link to the complete MC14 and MC13 change logs in my signature. The majority of the audio related fixes and enhancements in those change logs are included in MJ14 (naturally some of the listed things are related to "MC-only" features and are not relevant for MJ14).
The problem you have experienced is probably caused by one of the input plugins that extracts embedded cover art to the temp location on decoding, but does not delete it afterwards. What audio file formats do you mainly have? Is your cover art embedded (i.e. placed inside the audio file's tags)?
Regarding the temp location, I don't have MJ12 installed anymore, but at least in MJ14 you can set the location in Tools > Options > File Location > Program Files > Temporary files. Though, I have not tested if all included components actually use the defined location. I think its main purpose is to allow to set the location of the temp ISO file that is created when a data CD or DVD is burned with file format conversion. The size of this file can be up to about 8 GB per disc (dual-layer DVD).