When adding an image to reports or dashboards via the Textbox Widget, you'll need a direct link to the image. There are a number of free image hosting services out there (like imgur.com), you can host images on your own server, or use Google Drive.
Alternatively, you can use an image widget, and upload the image directly to your accounts media gallery from your computer.
How to add images to reports and custom dashboards
Once you've added a textbox widget, click the ellipsis "..." at the top right corner of the textbox and click on "edit" to start editing the widget.
Next, click the "Add Image" button at the top of the textbox widget.
In the Image Properties popup, add the URL to the image in the URL box.
Note that the link you copy into the URL field must end with .jpg or .png.
If the image doesn't display, check that you're not linking to the webpage that contains the image, rather than to the image itself.
To find that image URL, go to the page where the image is being hosted (in our example, this is https://imgur.com/CkGT6aP). Then right-click on the image, and select "Copy Image Address".
Ensure the URL for the image ends in ".jpg" or ".png" (and delete any text after .jpg or .png if you see any).
Once you've added your image URL, click "OK".
You should see your image in the textbox. You can then continue editing the textbox, or click the ellipsis "..." at the top right corner of the textbox and click on "done editing".