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imgur-integration is available in Gentoo's Sunrise overlay.
To access the overlay, you will need to install app-portage/layman, and add sunrise to your list of overlays. You can find more information about layman here.
Once you have layman installed (and make.conf configured to use layman), add the sunrise overlay:
$ sudo layman -a sunrise
Now you can install the package:
$ sudo emerge -av x11-plugins/imgur-integration