No indication of image loading when switching tags on slow connection

[kevinansfield]

Whilst proxying through ngrok to an iPhone I've noticed some odd behaviour with tag images appearing sticky. Steps to reproduce:

1. Create a number of tags with cover images
2. Ensure you're on a slow connection or artificially crippled network, clear your cache and refresh the admin page
3. Load the tags management screen
4. Browse through the tags without giving the cover images time to start loading/fully load
5. For the tags that have images you should experience the previous tags image still being visible despite the rest of the form changing, sometimes the image gets completely stuck until you visit another tag then go back. Clicking on to a tag with no image will immediately clear the preview.

Expected behaviour:

Image previews are immediately cleared when loading a new tag, potentially with some form of loading indicator if the new tag has an image.