Change album's main picture in Photos
Tap the person in the People album. Tap Select. Tap Show Faces to focus on just the person’s face in each photo. Tap the photo that you want to use. Tap the Share Extension, then > Set Key Face.
On your iPhone or iPad, click open an album you've created (won't work on albums iOS creates). Click Select in the upper right corner. Hold down the picture you want as your cover photo, until it "moves" or gets slightly bigger. Then slide it to the upper left position (first picture). Click Done and it's saved as the cover picture.
I don’t beleive there is any way to choose the key photo directly on the iPhone. There also isn't a way to perform this selection on Photos web app at https://icloud.com
If you are using iCloud photo library and have a Mac, the Photos app on your computer show the same albums as your phone. Open the album your interested in changing the cover photo for, right-click (or control-click) the photo you want to use and select ‘make key photo’. Once the changes sync up to the cloud, this change should be reflected on the phone when it syncs down those changes.
Also complicating things on iOS, there are two kinds of albums, those created by iOS itself (Selfies, Panoramas, etc), and then there are those albums you create yourself.
In those albums created by iOS, the most recent picture will be the so-called cover photo.
With an album you create, the cover photo will be the first picture in the album.
On my iPhone, I can drag the photos around to rearrange their order. Following the advice from above:
- Open the album you wish to change the cover of (this method only works for albums you created yourself, does not work for default ones like Selfies)
- Tap select at the top right corner
- Perform a long tap (tap and hold) on the photo you wish to make your cover photo
- Then if you drag your favorite photo so that it is first in the list, it will be used as that album's cover photo