Upl.co has had periods of downtime in the past, only to return as strong as ever, so it might yet return.
Most people around here have switched to using ImgBB for uploading images:
https://imgbb.com/
(Annoyingly though, I know of no alternative service for those of us who used Upl.co to display pdf files, such as my occasional crosswords, directly within a web page. We now need to use general file-sharing services instead).
ImgBB is a moderated service, so images which are deemed unsuitable (either because of their specific content or simply for copyright reasons) can sometimes disappear. Everything else should remain there for ages though.
Another alternative (although slightly less intuitive in my opinion) is Imgur:
https://imgur.com/
Click 'New post'
Click 'Choose photo/video'
Navigate to the location of your image file and double click on it.
Hover your cursor over the image and click on the three dots.
Select 'Get share link'.
Next to 'Share link', click on 'Copy link'
Paste your link into an AB post, preferably mentioning that clicking on the image will enable it to be enlarged to full size.
(If you want a link that
directly displays the image at full-page size though, you can use the 'normal' link yourself, then click on the image to enlarge it, followed by right-clicking and choosing 'Copy image location'. i.e. instead of this
https://imgur.com/godpqBo you'll get this
https://i.imgur.com/godpqBo.jpg )