Every since I installed Yosemite my background desktop photos change. I do not have the block checked to change them every 30 minutes, nor do they change every 30 minutes. They only change each time I boot up my iMac, which is every morning. I have an iMac mid 2010 model. I have a 27" Apple monitor attached to the iMac, using it as my main monitor. I have not moved nor changed any photo files. Everyday that I turn on my iMac the photos have changed from what i left them. It is a photo that was used maybe 6-7 months ago. Sometimes this photo will show up as only the iMac background, other times it will show up as backgrounds on both monitors. Then some days my iMac background will stay the same as it should. The 2nd monitor changes back to this one older photo every day. I have looked throughout this forum, under the Yosemite and iMac sections for an answer but haven't found anyone else with the same problem. Any ideas about what is taking place and a possible solution?