Whether you're creating a completely new post or reblogging somebody else's and adding your own two cents, an animated GIF can get your point across when words feel inadequate or simply fail you. Sometimes, however, you'll notice the GIF turns into a boxy gray icon on the dashboard once you've published the post and only appears when the icon is clicked. This happens to any image that isn't hosted on Tumblr's server -- but the post editor for most post types only allows linking to images from an external host. To get around this, you need to use the one post type that allows for uploading of images to Tumblr: the text post.
Open your Tumblr dashboard and start composing a new text post.
Click the "Upload Image" button, navigate to the location of the GIF on your hard drive and double-click it to upload it to Tumblr.
Click "HTML" to switch the editor to showing the raw HTML code for the post. You'll see a HTML tag that looks like this, with long strings of letters and numbers instead of "example:"
Select and copy the URL in the tag -- this is the URL to your image, newly uploaded to Tumblr's servers.
Click "HTML" again to switch the editor back to the normal mode. Click the arrow next to the "Post" button, select "Save as Draft" and then click "Save Draft" to save the post to your drafts so you can get the URL for the GIF again without having to reupload it if you ever want to use it again.
Open the post to which you want to add the GIF.
Click the "Insert / Edit Image" button, paste the URL you copied in the "Image URL" field and then click "Insert."