Tumblr automatically sets your uploaded image to be 500px wide - no matter how high resolution your image is. This is a known issue on Tumblr theme garden, and it's been around for years. Currently, there's no workaround to change your image resolution uploaded to Tumblr. Well, that's too bad, but don't worry about it. Use the following tricks
Getting back to fixing the URL structure of your image: on your custom page editor, open up the code view. (Click ‘HTML’ in your custom page editor). Find any image tag you would like to change. Your image may have a width data code '500', replace it with '1280' like the image link below.
// Here's the example. Change '500' to '1280'
You may be not a big fan of editing it every time when you upload an image, or not familiar with HTML at all. Don't worry. We have a script that works automatically for your custom pages. Place this code before </html>, or after </body> in your theme code.
This is it! :) Have any questions? Feel free to comment below!