Press 'Print Screen' in game, and go to you game folder to retrive the screenshots..
Then go to https:// simviation .com/phpupload/phUploader.php (No spaces) to upload images and copy the code it gives you into the thread.
Put the files you want to upload in a .zip folder, then got virustotal.com and upload there. Copy and paste the link to your thread also.
When creating the thread, click 'manage attachments' and upload the same .zip file there.
If your mod consists of a hex edit or sound mod, then you need to make a video and upload it to youtube