That will be an nice form control :-) and if you finished it you certainly need to put it in the marketplace.
The biggest question is where do you want to save the images? As attachments, in the Media Library or on a separate location. For example if you want use document attachments you can clone the "direct uploader" and look at the control "~/CMSModules/Content/Controls/Attachments/DocumentAttachments/DirectUploader.ascx" where the real work is done :-) You can also have an look at the API example in "~\CMS\CMSAPIExamples\Code\Documents\Attachments"
See also the Kentico documentation about developing formcontrols.
Good luck and if you have problems, let us known through Devnet so we can help you.
If this answer helped you, please vote for my answer :-)