There may be times when your .jpg image is not accepted into the media library despite meeting all known qualifications. It may be as simple as changing the file type.
In this example, I am saving a .jpg into a .png file but the same process still applies.
Find the image on your PC and right click to Edit with Paint 3D
Click on Menu then Save As
Click on the down arrow of "Save as type" to change the type from jpg to png. Then click Save.