Disk formats/auto-expanding FAT images?


While there seems to be significant information about using etcher to burn existing images, i am having a hard time finding out how to create images for use in Etcher and what image formats are specifically supported.

Obviously straight bit-for-bit “dd” style images are supported as well as various compressions on top of that, but are there any other formats? Particularly ones that would allow FAT32 image to be expanded to any USB drive while filling the entire capacity of the drive during the process of writing (to create a bootable USB drive that is also readable - basic MBR partition table with bootloader and 1 FAT32 partition filling the disk )? Or am I barking up the wrong tree here?