Firefox: Online image editor Picnik is pretty convenient, but you still have to manually upload image files or post their URLs to the site. Not so if you've got a new, Jetpack-powered extension that sends images to Picnik with a right-click.
It's an experimental extension, but even though it was built using Jetpack's no-restart-needed methodology, you don't need to have Mozilla's experimental platform installed to add Instant Image Edit with Picnik. After a standard install and restart, you'll see a new option when right-clicking images. It seems to just work, though there's one big caveat — the extension doesn't work behind password-protected sites, VPNs or other restricted pages. Other than that, it's a convenient tool for those who sometimes need just a bit of basic crop/colour/edit functionality.
Instant Image Edit with Picnik is a free add-on, should work wherever Firefox does.