How can I remove black frames around photos in the Photos app?

There are two ways to view picture thumbnails: you can see them in their correct aspect ratios, or you can see them with no spaces between. In the Library view and in Albums (and in some other places) you can switch between these two distributions by using the Sort menu and choosing View Options:

You show that you are using the Aspect Ratio Grid which has to leave room for horizontal as well as for vertical pictures. Square Grid leaves no spaces, but it has to cut off parts of the thumbnails to fit the grid. Personally, I use Aspect Ratio view most often, since I almost always take pictures horizontally, and I find square grid doesn't let me see the true form of the photo. But Square Grid lets you me more pictures at once, so it's good that I can quickly change views.

Is this what you needed?

This animated screenshot shows what Richard is referring to: either all square thumbnails, losing some of each photo or as aspect view showing thumbnails in their portrait or landscape orientation.