@mwenyi Ask yourself, "If I couldn't see this, what would I need to know about it to understand why it was posted?"
For example, a picture of a cat. Sometimes you need to describe the cat. Sometimes what the cat looks like isn't as important as what the cat is doing. Sometimes what's important is that it's a drawing of a cat, not a photo. Sometimes it's sufficient to say "a cat".
@ardgedee many thanks, this is a useful guide. I will remember