Currently there is no support for adding JPEG images to table cells.
It's possible to insert icons from the icon libraries that you have installed for your project. This is done using the {icon-name} wiki syntax. Note however you are limited to icons available which must be provided in vector format and not JPEG. See the user guide for more details:
http://wireframesketcher.com/help/help.html#working_with_screens/editing_text
If you wish to other kinds of images, then you can try to overlay the regular Image widget over the table.
If you can give me more details about the result you are trying to achieve, then perhaps I can give you more suggestions.