Thank you for writing.
Excel actually does not support inserting images into cells. You can see that if you insert an image into excel it will float over the cells and will not be inside any particular cell. Therefore there is no support in ExportToExcelML for exporting images.
On your second questions, currently this is also a not supported feature, but we believe it will be a good addition to the existing functionality and I have added a feature request to our Public Issue Tracking System - PITS. You can track its progress and add your vote/comment to it on the following link - PITS Feature.
I have also updated your Telerik points for the good suggestion.
Do not hesitate to write back with any further questions.
All the best,
the Telerik team