A bug fix has been applied to the Bundled theme - images for photo posts were being resized which was breaking animated images. The fix should automatically apply to theme users, but if you are still experiencing this bug you may need to do the following to correct the problem (do not do this if you have applied custom modifications to the blog template).
Go to Settings, click on the Blog tab and scroll down, click the Theme: customize button. Click the Edit HTML button. Make sure that Blog template is selected from the drop down menu, then click the Reset template button and then click the Save button.
If you have made your own custom modifications to the Blog template this bug fix will not automatically apply to your theme. Please click the read more button for step-by-step instructions to apply the bug fix. Apologies for the inconvenience!
Navigate to Settings > Blog Tab > Theme: Customize > Edit HTML and select the Blog template. Scroll down to around line 200 for the chunk of code for photos. Find the line that reads:
Replace the section underlined in red from photo.400 to photo.600
600 is the standard size code that should not mess with animated images nor should it be too large to disrupt the theme design.