Image cache for lots of images 2.0
Just a small thing: consider using object and collection initializers.

So this:

NameValueCollection config = new NameValueCollection();
config.Add("CacheMemoryLimitMegabytes", cacheMemoryLimitMegabytes.ToString());


Becomes (also using var):

var config = new NameValueCollection { { "CacheMemoryLimitMegabytes", cacheMemoryLimitMegabytes.ToString() } };


And this:

CacheItemPolicy policy = new CacheItemPolicy();
policy.SlidingExpiration = SlidingExpiration;


Becomes:

var policy = new CacheItemPolicy { SlidingExpiration = SlidingExpiration };


And if you do that, you might want to get rid of the variables, and just specify the value of the parameter directly.

Also, it's common to use camelCase, not PascalCase, for private fields.