For the moment, ScreenFlow doesn't do still screen captures. Here are some shortcuts for capturing your screen, and some super-shortcuts for getting those caps into Photoshop with lightning speed:

Apple+Shift+Control+3 - takes a grab of the whole screen.

Apple+Shift+Control+4 - gives you crosshairs to select your area.

Apple+Shift+Control+option+3 - takes a grab of the whole screeen and copies it to your clipboard for Apple+V'ing it into Photoshop to edit

Apple+Shift+Control+option+4 - gives you crosshairs to select your area and copies it to your clipboard for Apple+V'ing it into Photoshop to edit

Apple+Shift+Control+4 then spacebar then clicking lets you grab only the active window.

Apple+Shift+Control+option+4 then spacebar then clicking lets you copy only the active window to your clipboard. (That one's the powerjam!)