How to fix transparent photos and wallpaper issue on iPhone SE

I'm not exactly sure what you mean-- I don't think I see any transparency in pictures shown in Photos, but there are places that are supposed to allow some background to pleasantly (or annoyingly) bleed through. Try going to Settings, pulling down to reveal the search field, and enter "transparency." It should show you an option of "Reduce Transparency" that will take you to the Accessibility section where you can turn on "reduce transparency."

I'm usually not bothered by this, but every so often something looks weird, and it takes me a minute to figure out that its the background leaking through! Let us know if this helps....