Tumblr is among the most visual of micro-blogging sites with most posts -- even text posts -- including one or more images. There are, of course, several different methods to post images on Tumblr; which you use depends on whether you want to share a group of images, decorate your blog theme or add a funny image macro or reaction GIF to a post.
Among Tumblr's many post formats is one designed expressly for the sharing of images -- the photo post. Each photo post can include up to 10 images; when a photo post has more than one image in it, you'll also see it called a "photoset" or, if the images are animated GIFs, a "gifset." To create a new photo post, simply click the "Photo" button on your Tumblr dashboard. You can arrange the various photos in a photoset however you like; if you have a webcam, you can also take a photo of yourself or record an animated GIF.
On Your Theme
The easiest way of adding an image to your theme is to add it to your description. First, upload the image to an image-hosting service external to Tumblr (such as Imgur, ImageShack or TinyPic -- links in Resources) and copy the direct URL the service displays once the image has finished uploading. Click "Settings," click the name of the blog to which you want to add the image and then click "Customize" to open the options page for that blog's theme. Add the following code to the "Description" field, replacing "IMAGE_URL_HERE" with the URL to your image:
Click "Save" and then click "Exit" to save your changes and return to the dashboard. Keep in mind that an image added in this manner will end up wherever the description is on that particular theme, most likely in the sidebar. If you want to place the image elsewhere in the theme, you'll need to know or learn enough HTML to figure out exactly where the image code needs to be added to the theme code to make it show up where you want it.
In Text Posts
If you're creating or reblogging a text post, you can upload an image to Tumblr right from within the editor: just click the "Upload Image" button, navigate to the location of the image on your hard drive and double-click it. If the image is larger than Tumblr's stated restrictions (see Resources), it will automatically be resized and, if it's an animated GIF, may turn into a static image. If you don't want this to happen, you can also upload the image to an external image host, click "HTML" to switch the editor to raw HTML mode, and paste the same image code mentioned in the previous section where you want the image to appear.
In Other Post Types
You can also post images in the caption section of most other post types, including photo types, with the exception of chat posts. You will, however, need to upload them to an external service. Once you've done that, click the "Insert / Edit Image" button on the post editor, paste the URL to the image in the "Image URL" text field and then click "Insert."