Preview fix

The problem was discovered by @gusferguson while reviewing gh-2722
It was possible to have access to preview even for big images. This should not happen in 5.0.3
I have also fixed the tooltip of the tab, the text could still indicate the arbitrary value

To test the PR:
 * Import images from the jpeg folder if not already available. i.e. 2kx2k, 4kx4k
 * First select the 2kx2k. Go to the preview tab. The image will load.
 * Select the 4kx4k. Selected tab should switch back to the general one and the preview should not be accessible.