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Objects of type IMG_RESOURCE returns a reference to an image, possibly wrapped with stdWrap. Can for example be used for putting background images in tables or table-rows or to import an image in your own include-scripts.
The array $GLOBALS['TSFE']->lastImgResourceInfo is set to the info- array of the resulting image resource (if any) and contains width, height and so on (similar to how $GLOBALS['TSFE']->lastImageInfo does for the cObject IMAGE).
Depending on your use case you might prefer using the cObject IMAGE, which creates a complete img-tag.