There could be dozens of reasons actually but the most common reason is because the images used to display your blog are hosted on another site with limited or capped bandwidth. This is the only way template designers can build and store these images since Blogger is limited and you can’t store the images by default.
The Blogger template code is of course hosted and stored within the Blogger system but if you actually look at the .css code in the header of the template, you’ll see ALL the images are being pulled in from another location.
Example: Let’s say Joe, our template designer decides to build a new xml Blogger template about iPods. He hosts all the images on his free Photobucket account (which only offers 25GB/month of bandwidth) and then puts the template on his website so everyone can download it. Turns out his template is a hit and over 1,000 people download it in the first week.
Joe is happy but doesn’t realize that it’s only a matter of time before people start complaining. Once his 25GB/month limit hits, then those 1,000’s of people who are currently using his template will see something to this effect on their blogs.
Essentially for each image that’s on your blog, you’ll see the “Bandwidth Exceeded” message. It’s really quite an ugly-looking mess but that’s what you get when you hit your bandwidth limit on a free image hosting site. This is the snowball effect for a successful Blogger template which essentially is just a math equation.
25GB - total bandwidth used = bandwidth left for the month.
You of course have no idea what the total bandwidth used number is so you’re flying blind. Your template images might disappear tomorrow or you could be fine for years to come.
Did you know that the most common time for your images to disappear is at the end of the month? Most free image servers base your bandwidth limit at a monthly basis. I call this the “End of the Month Blogger Blues” when thousands of Blogger users panic and start asking Google Groups support “what happened to my images??“. They also start searching for a new template since they assume replacing it will solve their problem (which it usually does until the next template goes through the same growth problem).