I found out that this is generated by the wordpress classes for aligning images (or so). For me it’s a miracle, that someone can think anyone needs a brown-grey field with a dark grey border behind an Image.
The classes for aligning the images are part of all themes – it is required. But they are for alignnone, alignleft, etc.The only place that inline styling came from (the style= part) would have to be a manual entry purposely done by someone at some time. Is is not generated by the theme.