When you upload images into your gallery (through drag and drop or the file selector), you will see the uploader appear in the bottom right of the screen
While the upload is happening, you can still navigate around different galleries. You can even start multiple uploads on different galleries.
If your internet connection gets interrupted during an upload, the uploader will pause and wait for the connection to come back before continuing.
The uploader handles many different file formats. Learn about supported file formats.