ImageResizer is one of top choice library when it comes to optimizing media assets in web context. EPiServer 7.5 introduced really cool feature Asset and media.
You need to thank and follow Martin Pickering hard work to get both of these worlds working together which involves a bit of hacking here and there.
Recently I got sick of repeating these steps from project to project and came up with a package of NuGet.
Package gently modifies your web.config file in order to add new plugin to the list of ImageResizer plugins.