When you add pictures to a "Gallery" the frame size that you set will be the same for every image you add, so if you are adding images that are both portrait and landscape to fit within the same size frame you are going to end up with some of your images being cut off.
As you add each image to your "Gallery" you will see a drop down option saying "Crop from ....". If you click on this drop down list you will see that you can specify how the image is cropped, for example, "Crop from the centre" or "Crop from the bottom". This can often help your images to fit better within the frame.
If you are using a mixture of portrait and landscape images it would work best to have a different Gallery for each type, so you can then make the size something like 100 x 150 for the portrait image Gallery and 150 x 100 for the landscape image Gallery.