Hello,
I searched the forum and the web, but I cant find a solution.

I will distribute a image to install on a small computer with SD-Card.

The space I need is less as 2GB and I have 2 partitions. The FAT boot-Partition with UBoot and the root partion.

As I have only big HDDs or SD-Cards, I shrinked the partion sizes to 20MB and 1.9GB to fit on any 2GB SD-Card.

But how can I write down a image not from the whole SD-Card (i.e. 8GB) , instead only from the 1st and 2nd partition (less 2GB)?

The User should copy the image with Win32DiskImager to a SD-Card. Then, with a script - I will expand the filesystem or, if installed, copy the root system to a SSD and expand there.
(and, of sure, change the entry in env.txt)

dd count=<howmanyblocksyouwant>

I usually make a vm of this device with a smaller hard drive and file copy the data part with gparted or other file by file. Then either copy the loader or just add it in.