Use something like
Photo and image hosting, free photo galleries, photo editing and register as a user.
Click on Editor (top of page)
Drag thumbnail picture into editor window
Click upload
Click on resize and select 850 by 850 or smaller.
Apply
Save
Click on photo
Click on IMG which will copy the image
Go to the Prune message window, right click and select paste
In the future your settings will be remembered so you won't need to select Editor.