Q: Does this mean that one can upload photos, images and videos at Instagram only via a smartphone app and not via a desktop or a laptop?
A: No, you can use Desktop or Laptop as well to upload the image or photo from Instagram website / URL. Though you can only upload photo and video from Smartphone or Tab which has Instagram app but the Desktop or Laptop also can be used to just uploading the images than video. Please find the steps below;
How to upload the images in Insta from Laptop or Desktop?
Login to Insta from your Desktop / Laptop visiting its website
You will get a part screen (the code at right side and app at left side)
Right side top, click on a small 'mobile icon' which is before 'Elements' tab.
Left side, use 'Responsive' option in case if you know to select the device - default must be 'mobile' - leave as it is if it is mobile.
You can view Insta normally and upload just an image using or double clicking on 'Camera' icon which option available at the bottom of the Insta.
Q: Can you clarify this and if an app is required, let me know how to proceed further to submit an image there?
A: If you are willing to upload image from Smartphone then just click on or press on particular image and it will show various 'sharing option' including Gmail or other default apps to share it. In this case, select Insta icon to share it. Yes, you can adjust, resize or change the color and setting etc. beside writing some words while uploading it. You can do it the same from the folder of the Photo Album too.
If you are willing to upload photos from the Desktop or Laptop then just follow the steps given above and the computer folders will open upon clicking on the 'camera icon', so, proceed accordingly to upload the image.