Default ImageBase for UEFI images
As you guessed, it doesn't matter, since UEFI images are relocatable. Which is sort of a must since UEFI does not provide address-translation - all drivers and applications share the same virtual address space.
What is the image base (in windows PE files)
It is the address in virtual memory where the executable should be loaded at to avoid any adjustment of absolute jump instructions in the code.
The OS may load the module to a different address (in case e.g. 2 DLLs needed by the same program have the same image base), but in that case the code needs to be patched when loaded.
For more info, see https://msdn.microsoft.com/en-us/library/ms809762.aspx (search for ImageBase there).
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