Peter,
It depends.
If you don't get a lot of traffic from image search there is no need to do anything special.
If you are receiving a lot of traffic you basically have two options
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redirect the images to the new location (if this is easy to do)
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alternative would be to keep the images in the old location & put them in the new location as well. As you update the sitemap & redirect the url's Google will after a while update the index for image search & get the new location. As long the index is not updated, your 'old' images will still appear in image seach, when people click on the link they will however be redirected to your new page (as the page containing them is redirected). In my experience - the update frequency of the image index is lower than the normal search index.
I applied this strategy on 2 migrations of sites with 90% of search traffic = image & it proofed quite successful.
Hope this helps - let me know if you have additional questions,
rgds,
Dirk