A quote from ImageProcessor repo
We already have a MyGet package repository - for bleeding-edge / development NuGet releases.
The link points to some json file.
I used to install NuGet packages by id. What should I do with json?
If you are using Visual Studio you will need to open the Package Manager Settings and create a new package source with the MyGet url. There are two URLs, one if you are using NuGet 2, which is for Visual Studio 2013 and older, or another url for Visual Studio 2015 and NuGet 3.
Note that you can use the NuGet v2 url in Visual Studio 2015, it still works there.
From the Tools menu, select NuGet Package Manager, Package Manager Settings. Select Package Sources. Then click the plus button to add a new package source. Give it a name and use one of the above feed urls.
Then open the NuGet package manager itself and select the new package source you added. Note that you will need Include prerelease checked to see any NuGet packages for the image processor feed.
If you are using MonoDevelop or Xamarin Studio then use the NuGet v2 Feed url.