You can upload your pictures under "Media" when your route/track is in edit mode.
To do this, click on "+ Add photos or videos". You then have the option to choose whether you want to add a previously uploaded picture to the route/track or upload images from your computer. You can also link a YouTube video under "Select video".
To upload, select one or more images and click on "Open" to proceed.
You can choose one of your uploaded pictures as a thumbnail. Please note: The picture must be published, otherwise you cannot select it. This picture will then be displayed as the title picture of the route, for example in the route search. If you don't select any of your pictures as the thumbnail, the first picture of your gallery will be used as the thumbnail.