It's kind of like 6-of-one, half-dozen of another problem.
They kind of work out the same:
Separate image files will make the browser issue multiple
requests to the server. So that takes time and makes the server do
more work (cost is minimal on servers that are optimized for this
sort of thing).
A single large image still has to download the same number of
bytes, but in just one request. So it may be a tad faster. But then
you lose what you've gained by copying the pixels.
All in all, I'd use separate files and just save yourself the
hassle of dealing with the copy aspect. Your time is also a
resource. Plus, if you need to alter those images you'll have to