To use technical correctness, a ROM image is not a ROM. To distinguish them, a ROM is a hardware chip, but a ROM image is a computer file. An emulator uses the computer file to emulate the hardware chip. With the emulator, a ROM image acts like a ROM, so we say that a ROM image is a ROM.
By analogy, a tape image is a tape in a computer file; a disk image is a disk in a computer file. Each image looks like a ROM, tape, or disk but is not the actual hardware device.
ROM dump Edit
A ROM dump is a ROM image that is a copy of an original ROM. Every bit in a good ROM dump is identical to the matching bit in the original ROM.
After some hacking, the ROM image is not a copy of any actual hardware chip. This is the reason to prefer term "ROM image" over the term "ROM dump".
A ROM image is a sequence of one or more ROM banks. The ROM layout determines where the ROM banks will appear in the address space of the emulated system. Each ROM bank occupies some continuous part of the address space. From the ROM layout, we take a formula to convert ROM addresses.
A raw ROM image contains only the ROM banks. Some other ROM images contain extra information. Some systems have so many ROM layouts that the emulators need this extra information to select a layout. For example, NES image formats (like iNES and UNIF) store extra information allows the NES emulator to choose the correct NES mapper.
WLA DX can assemble and link a ROM image. WLA DX numbers the ROM banks in sequence, starting from 0. Thus an image of 4 banks would have bank 0, bank 1, bank 2 and bank 3. Each bank takes a slot in the address space. For systems with bank switching, WLA DX allows a bank to occupy more than one slot.