This is a ridiculous thing to do in the first place, since it would be much better to import the PNG as a project file and load it from an actual URL rather than a data URI. Even if for non-image content you have large amounts of data, it makes far more sense to paste them in to a text file, import them as a project file, and AJAX request them. You should never be pasting large amounts of data in to events.
However to prevent the dialogs disappearing/corrupting, I've implemented a maximum height for the next build. The dialog still won't display correctly, but you shouldn't be doing this in the first place.