As long as you're not breaking any copyright laws your images are legal to use.
Otherwise every news company on the planet would go bankrupt from lawsuits over copyright infringement.
To be on the safe side if you are worried about your own personal posts, you can give yourself some peace of mind.
Put a disclaimer in your posts stating you are not the owner and have no legal rights to the images in the post.
By using an image of or a link to a service or product isn't illegal.
If you look at help tips from this site alone, they suggest using images and links to help your readers understand what you're explaining.
I hope this helps answer your question.
If you ever doubt or question something you want to post, it's probably best to not post it.