There are basically two ways to do this.
1.) Resize all images in Photoshop/PSE to meet your needs and then Import them into PrE.
2.) Resize one image in the Timeline, Rt-click and choose Copy, then Select all still images and Rt-click, choosing Paste Attributes. This is easier if all stills are one the same Video Track - you just "lasso" them with the cursor.
I like #1, as PS has better resizing algorithms and controls, than does PrE, in my tests.
Thankyou, Winesnob. you made my job ever so much easier! Next round's on me...
One caveat that I should mention with regards to method #2: if you have added any Effects, that you do not want in the rest, to that first still, the one that you will Copy the Attrbutes from, remove them, as they too will be applied to the rest. Nearly all Effects can be applied in this way.