You have poor css.
Mobi/KF7 is HTML3 with a few extensions (there are other mobi formats). The azw3 is similar to epub, in that it is a subset of HTML5 and CSS3. So a conversion to mobi just has simpler inline styles.
Correctly done CSS for images in epub2 converts perfectly to epub3, mobi/KF7, azw3/KF8 and KFX. My experience of 11+ years of publishing and conversion using Calibre.
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