Tumblr automagically shrinks any images you upload in text posts to 500px and replaces images from other websites with an ugly grey icon, but here’s a workaround that will let you post images of any size!
Go to your dashboard and open a new photo post.
Select the photo you would like to post.
Upload the photo as a private post.
Click on the photo once the post finishes uploading and then right-click and copy the image URL.
Open a new text post or reblog a post and switch from rich text formatting to HTML (the far right button on your format bar). Use the following HTML code wherever you need your image:
<img src="PASTE IMAGE URL HERE"/>
Click Post and you’re finished! The images should show up as native Tumblr image thumbnails.
You can always delete the original photo posts once you’ve finished with them.
All images in this tutorial were uploaded in the same way. Let me know if you have any problems with this method!