Do you get al lot of traffic from image search? If not, I don't think it really matters, there are thousands of site who are using the same setup as you.
if you get a lot of traffic from image search you need to be careful. Never moved from main domain to sub domain, but did a recent migration of a site where 80% of traffic came from image search. All the images were moved to a new folder.
In my experience, the image index is updated much slower than the normal index, and while google is quite fast to index the redirected urls it seems to need much more time for the images. On a previous migration we had the situation where the image thumbnails where still visible, but as soon you did preview the image the image didn't load as it had moved from it's original location. The problem with image search is that when this happened the position of the image remained in the results the site behind it changed (at that point we also discovered that an huge number of sites had stolen our original images).
To avoid this we now migrated the images to the new location and updated the links in the html, but we also kept the same images in the old location until we had the impression that they were no longer indexed.
this strategy seemed to work as unlike the first migration our results from image search were unaffected.
hope this helps
Dirk