Save images as BMP but not wanting to

[Man of Steel]

My IE has got the nasty habit of SOMETIMES saving particular images as a BMP file. Most of the time on the same pic´s.
I already tried cleaning up my temp. Internet files, clearing cookies, setting my temp. Internet files to max 50 MB etc.
Sometimes when I do not open pic yet, but choose ´save target as´ it saves as JPEG as it should do (but again not always).
You can also hit control C, open Paint, control V and save as JPEG (or via Irfanview etc) but is very time consuming. Who could help to get rid of this problem?