Here are some pointers.
- Thumbnail images have to be separate from original (large) images. When the user uploads the images, you have to use some script to resize the images. If you are using a standard CMS like Drupal or Wordpress, there should be an option somewhere to do the resizing (without you having to write code).
- Assuming your blog is public, the images as well should be public (usually). You can create a directory named files and you can store the images inside that directory. If you are using a standard CMS, these options should be there in some form.
- To avoid having all files in one directory in the long run, use folder naming schemes like files/[YEAR]/[MONTH] or anything else you think would serve your purpose.
- Make sure the uploads directory and your upload mechanism is well-protected using and
.htaccess (or equivalent). Otherwise, someone might upload malicious scripts and execute them on your server.
- A placeholder is anything which holds the place of something while the original thing is absent (or being loaded). So, a placeholder image will be a standard image with a general design - it's as good as saying loading. You can use JavaScript or CSS (background-image) to achieve such a placeholder.
- Preloading should not be necessary as far as I see from your question. A better opinion / answer could be given if you share the link to your site.
Next time, please try to make detailed questions - one question per problem, if possible. Also, do not fear to Google for a solution. I learnt programming (PHP, JS, Drupal, CodeIgniter and more) just by Googling! Hope this helps!
