It is fairly easy (note that the english words might be slightly different; I am trying to translate it back to english from Czech PowerPoint. Damned localizations, hate 'em):
Menu: Insert -> Picture -> New photoalbum -> File or diskette
choose all the pictures you want to add and confirm
make sure that layout is set to "resize to slide"
click create
Fairly easy and exactly what I needed!! No need to resize images manually etc.!