drupal 8 image dialog position
Im not able to reproduce it on my drupal 8 projects. It might come from your front-end theme. Try to use an administrative theme for edition, like seven.

{yoursite}/admin/appearance
At the bottom of the page check use administrative theme for editing.

If the issue doesn't occur anymore on theme seven, you should start by checking in your front-end (custom?) theme for styles on thoses classes :

ui-dialog ui-widget ui-widget-content ui-corner-all ui-front editor-image-dialog ui-dialog--narrow ui-dialog-buttons