As you have discovered probably by now, you cannot paste photos into the message text box here on DU. Hey, no worry! Even experienced DU members with thousands of posts have to sometimes ask this question, so... welcome!
For a photo (image, video, etc.) to display on DU, the hyperlink (also called the URL or web address among other things...) must be placed in the message box. The image will display when you click Preview (always test!) and when you post it.
I want to post an image I found on the internet.
Depending on which internet browser you use, you may see different options when you right-click on an image you have found while browsing.
If you use Chrome, a right click on the image will display Copy image address.
Then paste the copied address into the DU message box. Bam!
If you use Internet Explorer/Bing, right click the image, click Properties, and you will see Address (URL). Select the address (htt p://blah blah...jpg (or png, gif, etc.) and copy it. Paste it into the message text box on DU.
If you use a different browser, Google how to find the image URL for that browser. They are all slightly different.
I want to post a photo that I have on my own computer. It is NOT on the internet (yet!)
First, get that image from your computer uploaded to a web-based image hosting site. There are many. Photobucket was very popular until just recently when it became unaffordable. Imgur is a free site. There are more explained in this document (valid as of 8/2/2017) https://www.lifewire.com/free-image-hosting-sites-3486329 .
Because of the different places to put your image, I can't give you step by step on how to upload your image. Once you have uploaded your image to your upload site, you will see that it may offer you various types of links. You need the Direct Link, as indicated in the screenshot below. Click the copy button. Then paste into your DU message box. Done!
But it just shows the URL in the DU message box and not the photo!
The image you are trying to link to may be part of a slide show or gallery. If you don't see the copy image address or Properties/Address/URL, it can't be done this way. You will have to get a screenshot of it and save it on your computer as a jpg file (or png) and upload it to the image hosting site, as shown above.
The URL you pasted into DU must end with one of the standard file format extensions such as .jpg, .gif, .png, etc.
What about YouTube videos?
Easiest way is when you are on the YouTube page viewing your video, simply click in the address bar for the page and copy that URL. Then paste it into the DU message box. See screenshot: