Make image upload dialog narrower.

Take a step toward fitting on small screens. (#566)
Show a blank image before one is uploaded.

Change the 'No Image Uploaded' image to a blank one. The blank
space should be enough to suggest that a thumbnail will appear
there. This will also avoid any confusion in the message of the
image, as what was really meant by 'No Image Uploaded' was 'No
Image Loaded' (upload is the second step). (#566)
Adjust size and layout of image upload dialog.

Make the dialog and buttons a bit narrower to further help
fit on small screens, and space the buttons a bit. (#566)