Annotable

Annotable is the most feature-rich "N-in-One" image annotation tool.
Recommended by Apple many times:
● Best of 2016
● Best App of May
● New App
● Best Photo Editing App
● Best Productivity App
● Best Tool App
"I’ve finally found the best all-in-one image annotation tool for iOS – Annotable for iPhone and iPad." —— Federico Viticci of MacStories
Features (many are free, some need to be unlocked via in-app purchases):
◉ Various common tools: rectangle, circle, arrow, line, text.
◉ Full customization: color, size, outline, font...
◉ Elegantly cover private information: can be blurred or pixelated.
◉ Or vice versa, highlight certain areas with a spotlight.
◉ If you really want to focus on one point, use a magnifying glass.
◉ You can even mark the text on screenshots of apps like Notes, Weibo (remember to use MoKe:), Safari, etc., just like a highlighter. It feels amazing and very practical, especially if you've had a similar experience: You're about to share a screenshot of a web page and realize you should have highlighted some text, but you forgot to select it with the cursor when taking the screenshot and the web page has already closed...
◉ Perfectly supports action extensions, photo editing extensions, iCloud Photo Library editing and synchronization, URL Schemes, 3D Touch, wide color gamut... Annotable makes the most of the powerful iOS system.
「Annotable fully deserves the title of 'ultimate image annotation tool' in terms of functional experience. With it, you can really delete the folder called 'Image Annotation' on your desktop.」—— sspai.com, a minority
Differences from many other similar apps:
◆ All result images are high-definition.
◆ Supports all images, not just screenshot images.
◆ Large-sized images can be directly annotated without having to crop and shrink them first.