“It would be best if you would have an option to create a thumbnail or not, because thumbnails creates redundancy when you’re not needing them.”
This doesn’t makes sense – you DO have the option.
When you upload an image, you then click on the image in the image toolbar.
Then you see a little menu pop up.
The first line says “Using Thumbnail” – if you click on it, it then says “Using Original”.
Keep clicking that first line until you decide which of the two options you wish to use. Make sure it SAYS the one you want to have… thumbnail or original.
Click “Send to editor.”
You will see either a thumbnail or the original size image in your text editor. Finish your post, and hit “publish” – this is all defaulted into WordPress, there’s no tweaking or additional plugins needed for this…