Mobipocket reader blowing up the image

Hello,
English is not my first language, so sorry if I sound weird. I am trying to learn to make a mobi file with the help of Jutoh. I don't know any coding yet, but would like to use media queries to tweak the image sizing for kf7 and kf8. As Jutoh does not allow tweaking the code directly (and apparently has no solution for image sizing for kf7) I figured out this convoluted workflow:

1. compiling a .mobi in Jutoh, omitting any image size specification
2. importing this mobi to Sigil with the Kindleimport plugin
3. using the Sigil plugin AddKindleMediaQueries
4. specifying in Sigil the size of the image both for KF7 and KF8 -- and saving as the final .epub
5. converting this epub to mobi with Kindlegen
6. testing how kf7 and kf8 behave (here I use Calibre's KindleUnpack, then check each kindle version. And I also use the ancient Mobipocketreader app on my computer).

I was thrilled to find out that Calibre showed both kf7 and kf8 obeying the media query size specifications. BUT Mobipocketreader (kf7) didn't. It showed the image at 100% width instead of the specified 100px as it should.

Does anybody have an idea why this is?