Neither mxmlc nor compc seem to have a -frame option. They do however have a -framework option.
This seems to be a Flex question. Please try posting to the Flex forums here http://forums.adobe.com/community/flex/flex_general_discussion for better response.
The frame option specifies to which frame in the SWF, the class goes.
Flex apps by default have two frames: the preloader and the actual app. You can choose to put data or modules in subsequent frames to take advantage of the streaming nature of SWF download/execution.
For an example, try downloading Alex's project here and looking at the compiler arguments/code: http://blogs.adobe.com/aharui/2008/09/using_the_flex_builder_3x_prof.html
Alex uses --frame=Three,Frame3Assets to put images in the app in the third frame.
Thanks Anirudh, that helps.