Chas and Nev have a good thread on the topic here:
https://www.ukworkshop.co.uk/forums/picture-posting-guide-t63716.html
I find it's better to upload images to a 3rd party host such as
Imgur. You won't be restricted to a certain file size of the image so you can provide really crisp, large photographs of what you're trying to sell for clarity.
1: Upload your Image to Imgur
2: Once uploaded you can edit your image to make it smaller, crop it or rotate it if you need to.
3: Once any editing has been completed and saved, right click on the image and click "Copy Image Location" which will look like this:
https://i.imgur.com/ev441JQ.jpg?1
4: Once the link is copied, go back to the forum and click the little button on top of the textbox called "insert image" it looks like a mountain range with a sun.
5. Once that comes up, paste the link to your image and click OK. It will look like this in the text box: https://i.imgur.com/ev441JQ.jpg?1[/img ] <- without the space I've added in the brackets just to make sure it doesn't post the image.
Once that's all done you should have something like this:
[img]https://i.imgur.com/ev441JQ.jpg