Have been experimenting with the many and varied options for placefloat to try to resolve this problem with no success. As per minimal example below I have nearly what I'm after - an image which floats to the right of a background box with a 1em offset - but I can't get rid of the white box which appears above it no matter what permutations I try. 

(Secondary query: why do I have to set leftoffset=, rightoffset= etc. individually for the text background? offset= seems to have no effect.)

Thanks for any help you can offer.

\definetextbackground[TintBoxedBackground]
        [background=color,
        backgroundcolor=cyan,
        frame=off,
        leftoffset=1em,
        rightoffset=1em,
        topoffset=1em,
        bottomoffset=1em,
        location=paragraph]

\definefloat[thematicimage][figure]
\setupfloat[thematicimage]

        [default=right,none},

        offset=1em,

        margin=0em,

        background=color,

        backgroundcolor=cyan,

        rightmargindistance=-1em]

\starttext

\starttextbackground[TintBoxedBackground]

{\bf Lorem ipsum dolor sit amet}\blank

\placethematicimage{}{\externalfigure[cow][width=0.30\makeupwidth]} Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum. 

\stoptextbackground

\stoptext
Duncan