I’d use printf to put the markup into place after the translation. You can force the order with the right modifiers, but translators are probably smart enough to not really need that.
printf( __('The image %sMUST%s be less than %s500 KB%s in size.', CUSTOM_TEXT_DOMAIN), '<em>', '</em>', '<strong>', '</strong>' ) ;
I consider Otto’s post to be one of the definitive sources on how to translate strings in WordPress and one of his “rules” is don’t translate HTML.