There are more sophisticated and detailed ways to do it, but this is the simplest, and at least it will keep the file size down, when you share the photo.
Pierre said he just downloaded GIMP, so I'm explaining how to do it with pictures from GIMP. The process in other graphics programs is basically the same. Just look for the same types of tools. Be sure to click the pictures as you go along, so you can see them larger and be able to read the tooltips.
First...bring up GIMP.
Choose Open File from the File menu, and then browse to the file you want to open. In this case, I opened Pierre's full-page picture.
Here it is, in GIMP.
From the icon menu at the left, choose the rectangular selection tool When you hover over the icons, it tells you what they are.
Starting at one corner, loosely surround the picture that you want to keep.
From the Image menu, choose Crop to Selection, and click it.
Voila! You now have a small picture. In this case, the picture could also be rotated about one degree clockwise, and more of the border could be removed, but it's not necessary. Just leaving it this way makes it very small and compact. Use the Save As option in the file menu, to save the picture with a new name (so you don't lose your original).
Here's the original picture (which really wasn't that large, file-size-wise (91.1 KB)...
Here's the quickly-cropped picture (note that I cropped it a little more closely than I showed above, but not much)...quite a bit smaller and easier to look at. Interestingly, because I saved it without compression, it's actually slightly larger in file-size (117 KB), which is OK. At least it fits on the page the way it should.
