The first thing you will see when you log into Spark is your project dashboard (or the dashboard of the last project you visited, if you have multiple projects). You can also access your this at any time by clicking Project > Dashboard in the left side menu.
This is a dynamic page, and the information contained therein is updated each time you visit. If you have not added a project image yet, you will see a prompt to add one in the banner area on this page. If you have added an image already, you can change it, as well as add a custom project color from Settings > Edit.