Before you can get started with InVision, you'll need to first create your mockups/wireframes in another design tool. Once those are ready, you can upload your screens and start adding interactivity, fixed headers and footers, overlays, etc. If you need a free online tool to create wireframes for your prototype, consider Pencil Project, wireframe.cc or moqups.com. This is also a great tool to validate your image sizes before uploading and testing on your iOS device. You can also check out material.io/devices 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.
Once your wireframes are ready, you can upload your screens and start adding interactivity, fixed headers and footers, overlays, etc.
NOTE: Please do not use actual end-customer personally identifiable information (PII) in your mockups (or otherwise provide such PII to InVision), except for the limited PII of users required for them to access the InVision service. While using our service, please do not provide us with any personal health information (e.g. information subject to HIPAA), personal financial information (e.g. data subject to PCI or the Gramm-Leach Bliley Act (GLBA)), or student education records subject to the Family Educational Rights and Privacy Act (FERPA). Generating realistic-looking dummy data can be done easily using our Craft Data plugin.