I downloaded a wallpaper app with HD wallpapers and was looking through it.. noticed my phone got a little warm after browsing for awhile. Does it normally get warm from high quality content? My friend told me the iPhone 6 runs warm sometimes cause when I felt his it was kind of warm too.
It can get warm... but nothing to be scared of. Oh, btw... It gets warm when the CPU is under stress, a wallpaper app is not very CPU intensive.
This is going to be sine give, but I often stream 2+ hours of HD video at a time and have never really felt the phone get noticeably warmer unless it's also on the charger at the same time. I wouldn't think loading pictures would be any different.