Collections in .NET do not "own" an object. So they cannot assume that the object isn't used anywhere else, they can thus not dispose the object. Ownership rules are entirely your own to implement. Which does imply that you also have to be sure the the image isn't, say, displayed in a PictureBox.
Ensuring the image doesn't occupy any memory anymore is iffy as well. You cannot manage memory yourself, it is the job of the garbage collector. However, Image uses a rather substantial amount of unmanaged memory to store the pixel data, that memory does get released when you call Dispose(). The managed part of Image stays in memory until the GC gets to it. It is small.
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