You can upload photos from within Flippr for Flickr, or from any photo app capable of sharing images, like the built-in Gallery app.
Select all the photos you want to upload to Flickr, then tap on the Upload button (upwards pointing arrow).
By default, you will be prompted to either choose a target photoset, or just upload to your photostream. You can disable this prompt from Settings - in this case, your photos will always go to your photostream only.