If you are running iOS 11, you can take and annotate photos direclty in the Notes app; which would let you keep them better organized than you can using the Photos app.
To take a photo in the notes app, tap the (+) icon and choose Take Photo or Video. To annoate a photo in the Notes app, double tap on the note in the photo. When it expands tap the pen icon.
Annotating a picture in the Notes app using the built in tools
If you want a more complete set of tools than those built into iOS, the Annotable app by Ling Wang is pretty good. It’s a worthy succesor of the Skitch app that LannyC mentioned. Using the Share sheet you can use Annotable to markup inside other apps, and in most cases the changes will be saved back to that app.This includes both Photos and Notes.
Annotating a screenshot in the Photos app usiing the Annotable app via the Share Sheet
There are a few other decent annotation apps, but they tend to be more specialized than Annotable. The only one I can remembe off the top of my head is PointOut. It specializes in highlighting and/or magnifying a portion of the photo, and has more powerful tools than Annotable along those lines.
Note: Skitch was purchased by Evernote. After they incorporated many of it’s features into Evernote, the app was abandoned. To be fair, they added quite a few features between it’s purchase and eventual demise.
