If you want to resize multiply images to a other (smaller) size you can use this command.
It's not recommend to enlarge pictures because that will have a negative effect on the quality of the enlarged image.
time find ./ -maxdepth 1 -iname '*.jpg' -exec convert '{}' -resize 500 '../big/{}' \;* time: gives you the time the command was busy. This is optional.
* -maxdepth option prevent find to dig lower then 1 dir. (see: man find)
Resize my photo's before upload
To convert my 12.8 Mega pixel photo's to a acceptale size for uploading to my gaallery I resize all pictures to 2304 pixels on the longest side.
find ./ -type f -iname '*.jpg' -exec convert '{}' -resize 2304 './small/{}' \;