Recently I happened to translate an article on SuMo, Mozilla's Support portal.
While I was working on it, I noticed, that there was an screenshot in English which should ideally be translated, too.
Since I went through the described steps myself, I took on the task to upload a screenshot from my computer and want to document the process. You can also find it in the knowledge base.
Go to the media gallery and upload the image there. Pay special attention to the name! Also, make sure to redact sensible information like your mail address.
After you've uploaded it, you can use it in an article by using
[[Image:Title of the image]].