I'm not sure what you mean by "adjusting the thumbnail"...if you want this because your photos end up not being cropped the way you want, then the solution is to crop your photos to a square before uploading them.
I automatically crop everything to a square and then I don't hae to worry.
This; a million times this. I have a 'template' that I use now to make sure each image comes out the way I want it, but it's a pain to do the guessing game. It isn't that hard to make it possible and it'd make a lot of people happy.
As a new seller, on dial-up to boot, I detest the guessing game of how to crop pictures properly. Trying to show details means a major close-up picture. It's easier to be able to slide your pic to the area that works best to rep your item. Other sites have this feature, so it's not an impossibility.
Yes, I have used this elsewhere and it is so easy to do with your mouse.
Etsy, please help us out...many of us are really just craftsmen. Computer and photography stuff, not so much our forte.
The easier you can make it for us, the better...and more helpful to selling.