If you are looking for a really quick & easy way to draw or write on a photo on your iPhone or iPad, there is an app made for just for you.
I love this app, because unlike many drawing apps, this app is super easy to use. In fact, I haven't seen an easier way to draw on pics.
Search for "Atom Draw" in the Apple App Store (Available in all countries). You can get it for iPad, or iPhone, or iPod touch.
Note: This is a paid app, because the drawing app has absolutely no Advertising or In-App Purchases. So that's a huge plus, as many people are happy to spend a couple dollars on a beautiful, ad-free drawing experience.
It's really so simple. Just press (+) to pick a photo from library, or take a new photo with the camera. Then draw on it!
Here's a 1-minute video tutorial.
To change the brush size, simply drag your finger up or down on the Atom Brush (the little dot in lower middle).
To choose any colour, tap the Atom Brush (The little dot in the middle) to open the menu, then choose the colour picker in the middle again. Now choose a colour, or drag your finger around in the palette.
Simply hit the share card to send your drawing on it's way! Hopefully this makes your life a little easier and more creative!