Imgur works for me as well ^^ Make sure when you put the image in the [img] code that you're using the "direct link" not the url of the image. It should end in .png, .jpg, or .gif!
I personally use Postimage. You can find the link to it here (https://postimages.org/)! You simply hit 'Choose images', upload your photo and it will bring you to the page with the links. Once there you will see "Direct link:". Hit the link beside that and it will copy it for you! Once you have done that head back to the forum and paste it in between [ img][ /img] (without the spaces of course) to close it.
I find Postimage to be reliable and there are no watermarks. <3 Let me know if this works for you.