So, for your images did you add an image as a plane?
An image as plane is only visible in rendered view or lookdev, not in viewport shading mode, which is where you want to be when modelling.
This would normally be used for adding distant detail in a scene, far off trees, clouds, a skyline for example.
Why not SHIFT + A > IMAGE > BACKGROUND. This will add your image aligned with view. Select the view, 1 – front, 3 – side etc. then add the image.
You will then have the control of the opacity of the image in the properties panel, as well as the position, depth (usually set to back), side (usually set to front). And you can see your image in viewport shading mode, which is I believe your issue.
