If yoy resize the portrait to fit landscape it doesn't look right.
Yep, it sure won't. Be sure you're not resizing and messing up the aspect of
the image...use the corners to resize, NEVER use the sides unless a
distorted image is your preference.
Once the top of the image is at the top of the slide, and the bottom of the
image is at the bottom of the slide when you resize using the corners, the
sides could be blank. That's it, that all you can do unless you crop the
image.
If you can't crop, the ONLY way to resolve this is to turn the image
sideways (90 degrees) on the slide, fit it as I just described, then have
someone turn the projector sideways during the presentation. The turn it
back for the next slide.
Or if you're using an overhead projector and transparencies, turn that
transparency sideways.
If you're printing presentations to paper, you can't turn a single page in a
handout. Well, you could, but that would look silly, wouldn't it?
John O