simple solution: right-click (on mac os ctrl-click) on the picture ➔ 'fit to frame' (or something alike – i use a german version, so i don't know the precise expressions…)
a better way (more controlls) is the properties-window (f2). on the 'image'-tab click 'free scale'.
pay attention at the link-chain for the aspect ratio – the chain should be closed when scaling…
for a scribus-beginner, the properties-window is a real gold pit, that is worth playing with on different objects…
utnik