After you have selected or droped the file you upload/send it. Then it should look like this:
- Spoiler:
Now you can either copy the first link-address, then Click on the "spoiler"-button, put the textmarker between the two spoiler-square-brackets and then click on the "Insert Image"-button and paste mentioned link-address in the url-field.
Or you can just set the textmarker to said position and click on the symbol (I guess it's two layered speech bubbles) at the beginning of the second address.
Or you copy the address from the second line and paste it between those spoiler-brackets.
I think your mistake was to just copy and paste the first link-address.