the full image won't help you with repartitioning. it's for backup purposes only. additionally, you will have to dump each partition one by one...
don't touch the first 2 partitions (the primary ones)!
you don't have to resize system partition. leave it as it is. app get installed to data partition.
data partition and reader partition do not have to be restored. simply format data to ext4 and reader to fat32. and give them the right labels (READER and data). uppon boot, the reader will re-initialize them.
all other partitions (system, fonts, etc.) are kept untouched. backup them one by one and then restore them one by one.
so the steps are:
1.) backup extended partitions (logical drives, mmcblk2p4...mmcblk10)
2.) delete extended partitions
3.) recreate extended partitions with the first one shrunken by the size you wanna enlarge data partition
4.) restore extended partitions
Be sure to check out the Wiki for Sony PRS-T1 Rooting and Tweaks!
Many questions are answered there already... If not - feel free to add
Some modifications will void warranty and may irreversibly destroy the device. I won't take responsibility if you damage your reader by using one of my files or following my suggestions.