Dieter,
I would recommend starting with getting the demo framework project to compile, deploy and load with the same open ui controls to verify you have all the groundwork in place and working. Then look at creating your own control.
I can't tell from your post if you are trying to create your own image capture control or are trying to post-process a captured image before uploading. JPG images are already compressed so you really only have few options to reduce the size.
1) Lower the resolution
2) Decrease the JPG quality
Simply trying to compress the image won't yield much in terms of space savings.
--Bill