Products go out of demand when trends change, as Christine says. I would not be afraid of changing the pics for this product as it is necessary to keep "in trend" with pics and styling. Note, this is as it is not selling anymore. If you are afraid/not comfortable changing it, maybe you can rearrange the order your pics are showing as a slow start of reworking your listing. After a week or two (depending on your amount of traffic) make a decision. If the sales have not jumped back, maybe it is time to revamp it. Head up though, if it is just a summer drop in sales then keep it as is. Soon your customers will be back at their computers!