Once you have chosen a Challenge that you would like to contribute to, scroll down to the text box that says 'Add your Idea here...'. Click on the text box and you will be able to start typing your Idea.
You can upload images from your device or you can attach videos from YouTube/Vimeo by using the 
'Insert Image' or 
'Insert Video' buttons in the Idea toolbar. You can also copy the web address of an image or YouTube/Vimeo video and paste it in to the text area and it will be embedded.
innoloop will automatically try to choose some key phrases or topics from your idea and add them to the Trending Topics, but if you want to choose your own phrases, you can select the text you wish to be used and then press the 
'Highlight key phrases' button in the toolbar. This will let you choose which words are shown in the Trending Topics and help others to see a quick summary of your Idea.
Once you have finished adding text or images and video to your Idea, press the 'Add your Idea' button.