Hi,
beatrice explained that some time ago but i will reanswer.
First of all, you need to upload your image on a site, i suggest you Imgur.
Then, you will need to right-click on the uploaded foto and choose "Copy Image adress"
After that, you can come back on Smogon, go on "Signature" and click on the gear in the top right corner.
View attachment 424852
By doing that, you enable the BB code and now you can digit that :
.
You then need to replace "link" with the link of the uploaded foto, then re-click on the mini gear and save the signature