The second is called Mockplus. It also offers a free and premium version, the free version includes quite a few features:

> - Support all platforms(iOS/Android/PC/Web)
> - Sketch and wireframe style
> - 200 components
> - 8 markup components
> - Visualized interaction design
> - 8 pre-designed interaction components
> - Various events and commands
> - Execute interaction sequentially & simultaneously
> - Interaction state
> - Masters
> - My favorites
> - Fast review
> - Scan QR code to preview app prototype on mobile
> - Auto backup
> - Batch modification
> - Page tag
> - Component remarks
> - Import & export component fragment
> - Import & export project fragment
> - Ruler and guidelines
> - Share prototypes
> - Shortcuts

This basically includes all of your requirements but if you want to export to PNG/JPG, you're going to have to buy the Pro version.

# [Mockplus][1] (freemium)

> If you are looking for a tool that makes you design easier and faster, you cannot go pass Mockplus.

> 1. Fast Interaction- Interactive design is easier than ever - The interactive design in Mockplus is fully visualized, that's
> WYSIWYG. With a simple drag-and-drop to build interactive prototypes
> effortlessly. A set of pre-designed components, including pop-up
> panel, stack panel, scroll box, sliding drawer and image carousel,
> make you create fully interactions faster and easier.
> 
> 2. Fast Design -Prototype like a Pro with extensive pre-designed components Mockplus comes with more than 400 icons and nearly 200
> components. Just drag these components into the workspace for a
> combination to prototype your app ideas within few minutes. Focus on
> design itself and no more efforts will be spent on making a component.
> 
> 3. Fast Previewing - Test on actual device instantly Scan the generated QR code to preview prototypes on the actual device
> instantly, no USB cable or remote publishing required. Carry and share
> your prototypes offline with teammates or clients in order to gather
> feedback. The cloud sync makes it faster and easier to preview
> prototype on mobile device.
> 
> 4. Fast Learn - Code-free and get started right away Just focus on design itself rather than the tool. You can get started easily without
> the risk of falling into the tool's slave by investing a lot of time
> and money on learning and training. With Mockplus, you don’t need to
> worry about programming knowledge any more, let alone coding.

> [![screenshot][2]][2]


  [1]: http://www.mockplus.com/
  [2]: https://i.sstatic.net/sYQx6.png