Neither calibre nor Sigil are real ereaders. They both use the Webkit browser rendering engine to view ebooks as though they are web pages (because there is no reason whatsoever to write an ereader just for previewing in an editor) ... so of course they will show your pictures correctly, just like any other browser.
No ereader engine that I am aware of will do animations of any kind. (Except for iBooks maybe because they enjoy working differently from the rest of the known universe. It would be just like them to do something stupid like this.)
No one has put any effort into caring to try to support animations in ebooks any more than they have tried to do so for paper books.
For good reason.
If you want animations make an app.