Adding another WP plugin, along with the added server load, is not necessarily the answer.

Here's a simple, offline solution that will work at least as well as a smish.it plugin and perhaps better:

http://luci.criosweb.ro/riot/

RIOT will process your pictures in a number of ways. I use it every day and it's superior to any software-based processing as I have full control over the quality of the output.

It also has a batch function to process a folder of pictures at once.

Don't add to server load if there's a good solution offline.