Linux/filesystem experts? Extending img partition

[wayfarer]

I want to make a RetroPi image with my settings, roms, etc. ready to go. I start with a copy of the latest img (call it custom.img), then using dd

dd if=/dev/zero bs=1M count=4644 >> ./custom.img

this adds 4.5G to the total file size. Then I open the image in parted and expand the main partition

parted custom.img
(parted) resizepart 2
End? 6681

When I print the new filesystem as viewed in parted, it looks perfect. Unfortunately, mounting custom.img shows the original partition size rather than my new partition size. Does anyone know why?

sudo mkdir /mnt/tmp
sudo mount -o loop,offset=63963136 custom.img /mnt/tmp

[columboscoat]

Looks like you are doing it wrong. Try it a different way, one that works