Hi Salim,
Sorry for the delay in responding. I've been away for a while.
Most of the time, issues related to copying code to a block have to do with incorrect URL.
The easiest way would be to include the FilterCodes plugin and following the related documentation found at https://github.com/michael-milette/moodle-local_contact/blob/master/README.md#creating-a-new-form
As for the block not showing up for students, that is not a Contact Form for Moodle issue. I suggest you start by adding simple text first to a block and get it to display for students. If you are having trouble with this, you could try configuring settings however it might be simpler to delete and re-create the block.
Once you have that working, then you can replace the content and insert your image. Note that you cannot just copy the URL from the editor. While editing an HTML block, you will only see a temporary URL which gets replaced as soon as you save the content. If you copied the URL from somewhere else within Moodle, it won't work.
Once the image is in place, highlight the image and use the Link tool to turn the image into a link. Point the link to the contact form.
Hope this answers your questions.
You might want to consider signing up for the next Learn Moodle Basics course offered for free at https://learn.moodle.net/. The next session will be starting on January 7th, 2019.
