In the Preferences you can set the still image default duration to any amount of frames.
If you set it for 1 frame the 1200 stills will give you a sequence of 48 seconds in PAL or 40 seconds in ntsc.
Best is to convert the stills to framesize. This can be done in Photoshop as a batch.
You also can scale the still down to framesize by scaling down one still, copiyng it in the timeline, selecting all the others and paste attributes. This will be a strain on you computer.