Adding a photo directly to iPhone disk
Following the advice in this forum thread, I've deleted the files (after backing up to the computer):


/User/Media/PhotoData/com.apple.photos.caches_metadata.plist 
/User/Media/PhotoData/Photos.sqlite  
/User/Media/PhotoData/PhotosAux.sqlite


After opening the Photos app again, it rebuilds the Library:


  


It's important to use the pattern IMG_0000.ext for the pictures file names:


0000 being numeric ID of our choosing
.ext is a JPG or PNG extension
we can add letters after the numeric ID to prevent conflicts with the device ID counter, ie, IMG_0041b.png, IMG_0041c.png, IMG_0041d.png.