Our motto for the day is "A Sign Up Saved, Is a Sign Up Earned" :-). With that in mind, lets get to it. First things first, it's best to remove your underwear and get some kickass music crankin' when you are doing JPEG compression! More importantly though, BACKUP YOUR CONTENT before you start!!! If the content you purchased is on CD, it's not a problem. However, if it's downloaded content that's just sitting on your hard drive, it would be wise to back it up to write-protectable media(CDR, floppy, zip disk etc) before you start making thumbs, compressing for file size etc. Just because you are compressing your new images for what you are doing now, doesn't mean you won't want the full size versions later and if you happen to batch process your thumbs and overwrite your large images...well you get the idea.
Your purchased content will vary widely in dimension and file size. I have seen as big as 2000x1300 and 1.5MB per pic!! That's just a little larger than what we would like :). Generally speaking, you want your pics to be roughly 500 pixels on the long side with a file size of 30kb or less. Thumbnails average around 110 pixels on the long side and around 3kb in file size. These numbers are just a guideline, different strokes for different folks. Some pics compress better than others. As a rule of thumb, if the sum of my image files for a 20 pic free site is under 500kb, then I can live with that although that would be pretty high for a TGP gallery that is going to be getting a ton of traffic. Use your judgement. If you would like a good comparison of how you can compress the file size of an image and still retain good image quality, Click Here. Ok put your underwear back on because we are going to do some math(taking them off really doesn't help anyways, I just wanted to see if you would do it ;-P).
Say you are doing a 20 pic free site and your images are not compressed very well. Lets say they are 60kb each which is not all that uncommon if you look around at some free sites. Over the course of a month that site gets 20,000 hits with all the pics downloaded each time. You are paying your host $3.00/GB.
If you had taken the time to compress your pics a little better you would have spent only half that $$ on bandwidth. That's like getting a sign-up for free. This is just an example and doesn't include your thumbs or HTML pages but keep in mind that this is for one site only. There is a multiplication factor involved once you get building more sites and it really starts to add up.
It would be good to check the file sizes on the banners you are using as well. Some sponsor banners can be upwards of 30k or more which is very large. A word of caution however. The banners look the way they do for a reason, to get clicks and make you and your sponsors $$$. DO NOT degrade the image quality of your sponsor's banners and really it would be best to check with them before you modify their work in any way. It is possible to take a banner that's 20k and reduce the file size by almost half with no noticeable change in image quality.
By now you should have a good idea why it's important to compress your pics. I have a couple turorials based on two of the most popular programs that are recommended by the people at VNWR. Most people tend to prefer one or the other for different reasons. You can try them both for free so why not find out which one works best for you. Choose your weapon: