Since you have different ratios 1:1, 1.5:1 and 1.33:1.. you will either have to throw data away or create a canvas to paste the image on to.
Crop to fit:
convert <inFile> -resize 50x50^ -gravity center \
-extent 50x50 <outFile>
Fit within:
convert -size 50x50 xc:white \( <inFile> -resize 50x50 \) \
-gravity center -composite <outFile>