I would use automator, so you would create a workflow that looks something like this:
First, you insert a get selected finder items action, so that the computer knows what files to work with.
Next, add a duplicate finder items action, because we don't want to replace the originals!
Next, we add a rename finder items action, and set it to replace text in the drop down box at the top. This is because the perevious step appends the word "copy" to the name of each file it duplicates, but I don't want them to say copy, I want them to be the file name followed by an underscore followed by the letters "sm", which stands for small, because these are the small images. So in the find field, we type a space then the word copy, so it looks like this
Find: [ copy]. Then we type _sm in the replace field, so it looks like this
Then, we add a crop images action, because that is our final goal: to crop them into thumbnails. Make sure the drop down box at the top says "to dimensions". Then, enter the dimensions of your desired thumbnails in the appropriate fields. Then, before you click run, make sure to set the scale before crop drop down box to Scale to Short Side, so it looks like this
Scale before crop: [Scale to Short Side]. That way, the thumbnail will be a scaled down version of the image that fills the thumbnail box, rather than just being a tiny portion of the image.
Finally, find your images that you need thumbnails of in the finder, select all of them by either command clicking them or using the cmd+a shortcut to select them all, then press run!
There you go! You're all set with brand spankin new thumbnails.