First upload the image to OpenSlides: Open the Overview > Files menu on the left and select > Upload Files in the upper right corner.
Click on the file so that it is (automatically) displayed in a new browser window.
Now copy the entire URL of the image.
Open the overview of > Agenda in the menu on the left, select an entry and edit it by clicking on the pencil icon.
Click on the "Image icon" in the Text section > Paste/edit image > now paste the entire copied URL of the image in the "Source" field and save your entry(s).