Correct, no TRIM.
The impact on the SSD, in your scenario, would be gradual decay of write speed. Unless those are very, very big images you will never notice. What are you doing externally where an SSD's speed is needed?
Actually "nothing", just being obsessive. I built a PC with an Ivy Bridge 3570K CPU, more RAM than I really need and a SSD, so lots of speed at that side of things. Also, the ASUS motherboard has a USB 3.0 Boost feature which supposedly makes USB 3.0 faster (who knows?). Assume an image takes 17 minutes now, I want it to take 12 minutes. Total obsession but not a total waste of money as my Maxtor One Touch is probably five or six years old so it wouldn't hurt to have an external HD on hand. Thank you for the reply!