[Typo3] How to create a GIFBUILDER mask that is as large as the master image

[Christian Stuedemann]

Dear list,

a mask image for an imgResource (e.g. 100x100 pixels large) is always
enlarged to fit to the imgResource's size. I want to build a mask using
GIFBUILDER that is exactly as large as the imgResource so that it is not
resized before using it as a mask for the image. How can I find out the
dimensions of the imgResource automatically and set them for the
dimensions of the GIFBUILDER image?

Best regards,
Christian