Splitting them into directories sounds like a good idea. Basically (as you may know) the reason for this approach is that having too many files in one directory makes the directory index huge and causes the OS to take a long time to search through it; conversely, having too many levels of (in)direction (sorry, bad pun) means doing a lot of disk lookups for every file.
I would suggest splitting the files into one or two levels of directories - run some trials to see what works best. If there are several images among the 70,000 that are significantly more popular than the others, try putting all those into one directory so that the OS can use a cached directory index for them. Or in fact, you could even put the popular images into the root directory, like this:
...hopefully you see the pattern. On Linux, you could use hard links for the popular images (but not symlinks, that decreases efficiency AFAIK).
Also think about how people are going to be downloading the images. Is any individual client going to be requesting only a few images, or the whole set? Because in the latter case, it makes sense to create a TAR or ZIP archive file (or possibly several archive files) with the images in them, since transferring a few large files is more efficient than a lot of smaller ones.
P.S. I sort of got carried away in the theory but kquinn is right, you really do need to run some experiments to see what works best for you, and it's very possible that the difference will be insignficant.