Default for scale factor for images

Hi,

for my screenshots I typically add a scale factor of 40 to make the images size well.

<para>

		<mediaobject>

			<imageobject>

				<imagedata fileref="./images/application_model_ui_elements.png" scale="40" />

			</imageobject>

			<textobject>

				<phrase>Application Model UI Elements</phrase>

			</textobject>

		</mediaobject>

	</para>

Is there a general parameter for this? My desired behavior would be that I can define a default for all imageobject which do not specify this parameter but override it for individual imageobject via this parameter.

Best regards, Lars