Thanks for posting! :)
The optimum image size for your slider on the Brooklyn theme is 1200px wide.
As this theme is responsive, it will then automatically adjust the image size to fit computer and mobile screens alike. We have some more info on this in our Brooklyn Theme guide here.
Also some handy links to free image resizing and speed test tools below to help you out too:
I hope this helps!
Koreen W | Shopify
The Brooklyn theme keeps blowing my slideshow images up to fit the screen. I have tried using the image re-sizer link you provided above, exported the photos out of iphoto and set the max size, cropped them in iphoto setting the size to 1280X480... nothing works! I have spent hours on the forums trying to find an answer, but most people's questions are getting responses to change the HTML. Does this not have a simpler solution?? Thanks so much for your help!!