Assuming we're taking about images brought in via File:Place or by dragging and dropping a file from the Finder/Explorer, the resizing should work the same regardless of whether the image is in a trigger or not.
When a graphic is in a trigger container in a widget, and first click to select will show "Widget" in the Control Strip. The second will show "Container" for the trigger container. The third will show "Image Frame" for the bounding/cropping rectangle around the image. From there a double click will select "Image" which can then be resize or moved within the cropping rectangle.
To resize an image without changing its crop, click until the "Image Frame" is selecting, then grab a selection handle and resize. To change the crop, you can use the same approach, but hold down the Command/Control key while moving the selection handle.
Hopefully that's helpful? Perhaps I'm not understanding the question/confusion?
One nuance with objects inside a Container is that the container will resize to be large enough to fully encampass a contained object that is resized larger, but the Container will not automatically resize smaller when a contained object is made smaller.
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