Looking for a basic image editor with one special feature

Hello! First post as a roughly 1 year old switcher. After my 5 year old 15 inch IBM ThinkPad died last summer I decided to get a 17 inch MacBook Pro. One of the best purchases I've ever made! Anyway, here's my question:

Cropping images to a precise pixel dimension is something I do numerous times per day. My current workflow, which I perform in the built in Preview app, involves importing new images from my clipboard, then thanks to a hotkey I setup I quickly set the proper width of said image, then I create a selection box that has to then be manually dragged until it's an exact number of pixels high, hit the crop hot key, hit save, select JPG, and boom, I'm done.

I'm looking for an application that would automate some of these tasks. First, if there's a terminal command or something I can do to make Preview want to save images in JPG as default rather than PNG, that would be a lifesaver! But on to my main request: I'd absolutely love it if there was an app out there where I could hit a hotkey and then have a selection box of a a custom set number of pixels tall and wide appear on the image currently open. From that point on I could position the selection box where I want to, crop it, and then save it as a JPG file.

Thanks in advance! Freeware preferred, but I don't mind spending too much money. Please don't tell me to buy Photoshop, that's far too advanced, and costly, for my needs.

 

Thanks everyone, will give it a try! Any other suggestions besides GIMP?