When clicking on Show Images, the size of the image is not preserved (from original html in email, atleast with width and height attributes [not sure about styles]), The size is correct before pressing show images button, The code below (from scp/js/thread.js) is removing the attributes and setting style to auto weather they were defined beforehand or not.

.width('auto')
.height('auto')
.removeAttr('width')
.removeAttr('height');

maybe adding an original_width / original_height as attributes then restoring those if they exist instead of clearing them, or checking some flag if default widths and height were applied to not-downloaded images