The most common reason for not being able to uploading images in Magento 1 is that your flash is not enabled in your browser.
Magento 1 uses Flash to upload product images, and most modern browsers have, by default, Flash disabled. So if it was all working fine for you and suddenly you see no upload buttons anymore, this is most likely the problem.
This is normally an issue in Chrome, so I have added the instructions of how to fix this.
ENABLE FLASH IN CHROME
1. In the top right hand corner of your chrome browser click on the 3 dots
In the dropdown, click on ‘settings’
At the bottom of the new screen, scroll all the way down to the bottom and hit the ‘advanced’ word – this will open a new screen.