Most of ScreenToss’s functionality is accessible via its icon.
By default, ScreenToss will also upload screen shots taken the traditional OS X way: using ⇧⌘4 (Shift+Command+4). When the cursor turns from an arrow into a crosshair, you can select any part of the screen and it will be captured. You can use the space bar to toggle between selecting with the crosshair and selecting individual windows (the cursor will turn into a camera icon), or use the escape key to cancel.
You can drag and drop items onto the ScreenToss icon to upload it to your service of choice. Dropbox supports any file and many other formats, as well (including draggable images and text). Imgur only supports images and image files.