I purchased images from clipart.com, some of which came in both jpg and gif format. The gif format has a much smaller file size than the jpg format. Most of these pictures contain many colors and many curvy shapes (they are not images with straight lines and few colors). I had thought that pictures with many colors and curvy lines should be in the jpg format, and pictures with straight lines and few colors should be in the gif format.
All these images will have to be cropped and then resized to a smaller size.
I am somewhat confused. Should I use the jpg or gif formats?
You can see the pictures I am talking about here: http://betohelp.eu/. (This page has pictures with somewhat large file sizes.) The second set of pictures (the tree) is a perfect example - the jpg is 208 KB and the gif is 87 KB.
The graphics on those pages that have both jpg and gif versions are in fact quite simple graphics (less than 256 colours) which would be more suited to the gif version. When using something like Photoshop to optimise graphics, it may be possible to reduce the filesize of the jpg version to less than the gif version without noticable quality loss - sometimes you have to try both formats to see which is the smallest. With PaintShop Pro, I was able to reduce the tree jpg down to 69kB without noticable loss.
With photographs or more complex graphics, the gif format would not allow the picture quality required.
There are just a few images in there (the clay pots) that are actual photos. Those you probably should leave as jpeg. "lots of curvy shapes" is not the distinction you should be making for jpg vs. gif, its the number of colors in the image, as Centauri says.