Inexperienced website designers often place images on their site that are huge files. These pictures take a long time to load, and slow down the overall performance of your site. Visitors become frustrated waiting for your pages to load and leave for a more visitor friendly site. Pictures should be resized for display on the web. There are several programs that do this. Gimp is a free photo editing program similar to Photoshop. It is a little difficult to learn but allows you lots of creativity. If resizing your images is the only thing you need, there are many simple and free programs out there that can get the job done. A good target file size for images is the 20 to 50 kb range.
Some of us like to play with fonts. We like to mix and match and we aren't happy to use the handful that come with the template. This can create several problems. First, search engines are looking for your heading tags, and if you don't use them because you are trying to use a variety of fonts, search engines won't know what is important on your site, and you won't rank well. For more on that read about search engine optimization.
Second, your site can lack a uniform look. If you have different fonts for every page, it can get confusing and visitors won't know how to determine what is important. Finally, since Joomla is a content management system, it is fairly easy to do a site redesign by changing templates. However, if you haven't used heading tags, and instead used a series of crazy fonts, your redesign will be much more difficult. Your fonts may not fit with your new template and you will have to go back and change them all one at a time.
Pictures convey feeling. What do you want your visitors to feel? Look at your pictures. Do they express what you want? Pictures of large groups, pictures that are blurry, or under/over exposed don't get the job done. Be picky about which pictures you choose to place on your site. They communicate powerfully. If you can't find good quality images to go with your topics check out sites like istockphoto.com or Shutterstock.
It should be easy for your visitors to find what they are looking for quickly. If they don't there are plenty of other options out there, and they will click out of your site and look for another one. Also if you want visitors to do something on your site, such as purchase a product, that process ought to be simple and easy to access. Confusion creates missed opportunities, and leaves a bad taste in the mouth of your visitors.