Text on a website takes relatively little space. Pictures (and documents that contain pictures) can use a large amount of space, if the user is not careful when processing the pictures. The solution is to save a web version of the picture no larger than the largest size that will ever be used on the website as either a JPEG or PNG. Then, upload the web version of the picture, not the original picture.
A picture from today’s multi-megapixel cameras might be 20MB in size or larger. The largest picture on a website might be around 100KB. That means the picture from the camera is 200,000 times larger than necessary for a web page. This would be like driving a semi-trailer truck full of candy bars to a friend’s house, giving them one candy bar and driving away. It is an incredible waste of resources. It would be much more sensible to ride over to the friend’s house on a bike and give them one candy bar.
People unknowingly do the same thing when they upload huge pictures to a website. The web browser has to haul a semi-trailer truck of picture information to the user’s computer, just to display a candy-bar-sized picture on the web page. That makes the website appear very slow, because of the time it takes to transfer all of the information that won’t be used.
Fortunately, the WordPress programmers were aware of this problem. They designed the media upload feature to create three web-friendly versions of a picture when it is uploaded (small, medium and large). Go to the Dashboard and check Settings->Media for the specific sizes. The web-friendly versions are just the right amount of information to display the picture at that size, keeping your web pages fast.
If the uploaded picture is larger than the “large” web-friendly version, the original picture is just put “on the shelf” and not used, again. As a result, all of a site’s space could be quickly used by just a couple of huge pictures that will never be displayed on the website. The only way to get the space back is to delete the pictures, create much smaller versions and upload them, again.
As mentioned at the beginning, the solution is to save a web version of the picture with dimensions that are less than or equal to the largest picture you would use on your site. If your theme is 800px (pixels) wide, it is not useful to upload a wider picture. In fact, the picture should probably be less than 800px wide, because there will likely be a sidebar, margin or other item that takes up space on the page. A width of 600px might be a better choice for the largest picture. One exception would be a picture for the header area, which would cover the full 800px from one side of the page to the other.
If you create a PDF or other document with pictures, be sure to save a version “optimized for the web” or a similar phrase (check the software documentation). This will automatically remove unused picture information in the final document, making the document much smaller.