IrfanView, by Irfan Skiljan, is a freeware program that can handle the basic image manipulation needs for your HTML5 and CSS3 web pages and quite a bit more. Of course, you can use any software you want, but if something's really good and free, it's a great place to start.
A web developer needs to have an image manipulation program to help with all these chores. Like other web development tools, you can pay quite a bit for an image manipulation tool, but you don't have to. Your image tool should have at least the following capabilities:
Resizing: web pages require smaller images than printing on paper. You need a tool that allows you to resize your image to a specific size for web display.
Saving to different formats: There's a dizzying number of image formats available, but only a few formats work reliably on the web. You need a tool that can take images in a wide variety of formats and reliably switch it to a web-friendly format.
Cropping: You may want only a small part of the original picture. A cropping tool allows you to extract a rectangular region from an image.
Filters: You may find it necessary to modify your image in some way. You may want to reduce red-eye, lighten or darken your image, or adjust the colors. Sometimes, images can be improved with sharpen or blur filters, or more artistic filters, such as canvas or oil-painting tools.
Batch processing: You may have a number of images you want to work with at one time. A batch processing utility can perform an operation on a large number of images at once.
You may want some other capabilities, too, such as the ability to make composite images, images with transparency, and more powerful effects. You can use commercial tools or the excellent open-source program Gimp. IrfanView is good for basic processing, and Gimp is great when you need a little more power.
IrfanView is only available for Windows. Here are a few free alternatives if you want to try some other great software:
XnView: Similar to IrfanView, XnView allows you to preview and modify pictures in hundreds of formats, create thumbnails, and more. It's available for Mac and Linux.
Pixia: Pixia is a full-blown Windows-only graphic editor from Japan. Very powerful.
GimpShop: This is a version of Gimp modified to have menus like Photoshop.
Paint.NET: This is a powerful Windows-only Paint program.
Use Google or another search engine to locate any of these programs.