In case you want to use the default OS X keyboard shortcuts (⌘+⇧+3 for a full-screen and ⌘+⇧+4 for a selection-based screenshot), you first have to reassign different shortcut combinations in the Mac OS X Preferences.
Please note: Just disabling the Mac OS X default shortcuts does not set them free for Dropshare to use, you actually have to delete or reassign them.
To do so, open Preferences.app and navigate to Keyboard. Open Shortcuts and select Screen Shots (in older versions of Mac OS X, you might find the shortcuts under Services). You'll find the default keyboard shortcuts for capturing screenshots there. Assign new shortcuts to these functions, and Dropshare will be able to use the default keyboard shortcuts for uploading screenshots.