As a stop-gap, perhaps have a look at:
http://www.concrete5.org/marketplace/addons/picmonkey-image-editor/...

I find it's a pretty good replacement for cropping and basic edits.

Thanks for your prompt reply. The clients have been using PicMonkey quite happily but there now seems to be an issue that has only just started, where on 'Save' it reverts to 'Page Not Found' for some reason. I've sent a query off to PicMonkey and will wait for their reply.
It sounds as though the new image editor for 5.7 won't work in 5.6.1, is this the case?

I've experienced that exact Page Not Found issue with PicMonkey - it's nothing to do with them it's to do with the hosting.

I'd wager it's a 'Mod Security' rule on the server being triggered off (incorrectly) by the save back request.

Contact your host and ask them to investigate their modsec logs for any rules that are being triggered for the site and get them to whitelist the rule(s) that looks like it's related to the image save. If you trigger the error then put in your support request that can make it really easy for them to spot.

When the particular rule being triggered off is whitelisted the save should happen without any issue.