How to get ROM image from Android Rockchip RK3066 device

I have an rooted Android (4.4.2) device, mini PC (without screen or buttons) with Rockchip RK3066.

I was trying to create an image of its ROM using ROM manager, but when I tried to backup current ROM, it turned off and then said 'No command':

MiniPC doesn't have volume buttons so I was unable to go through this problem.

After that, I tried romdump using this tutorial, but although I have many files now, I do not clearly understand what should I do with all those files. My intention is to get update.img which I can flash to other device.

Any help would be appreciated.