Note that in order to get started, you must create your mockups/wireframes in another design tool. If you need a free online tool, consider Pencil Project, wireframe.cc or moqups.com. You won't be able to edit your wireframes from within InVision.
This is also a great tool to validate your image sizes before uploading and testing on your iOS device. You can also check out this site for popular device image sizing (including desktop/laptop monitors, click the upper right icon for monitors). You will need to edit your images using an external image editor (e.g. - Omnigraffle, Adobe Photoshop, Pencil Project, or Microsoft Paint) prior to uploading your image files in InVision.