Thanks for your responses y'all!
Regarding the crop tool:
Thanks for suggesting the crop tool! I haven't used it before but tried it out now.
Unfortunately it doesn't seem to fit my purpose as I'm applying a custom style to every picture which consists of a drop shadow and a border. When using the crop tool the shadow still works but the border doesn't as it's applied to the image that's being cropped and not to the cropped end result.
Also adjusting the cropping seems a bit tedious as it may involve enlarging the crop to show more of the image and afterwards shrinking the whole cropped image again to maintain the original dimensions.
Regarding when to convert to picture frames:
Is it possible to place images directly as picture frames?
My current workflow is:
Sorting through the images in another tool.
Placing all images relating to one topic at once in Publisher. (Usually I do this via Drag&Drop from my file manager)
Arranging the images (resize, rotate, remove some) - The photo book has a free layout therefore there aren't any specific layouts and placeholder frames that just need to be filled.