Here you are some basic guidelines to insert images on the web.
A main goal is get less size files as possible
Formats
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jpg:
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is used for photos (best than gif) - has a good compresion rate (best than png)
- can not manage transparency (gif can)
- is not a libre format
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gif
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is used in logos, draws, but not for photos (best than jpg) - is not a libre format
- can manage transparency (jpg can not)
- has a good compresion rate
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png
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is a libre (free²) format - has a worse compresion rate (than jpg)
- can manage transparency
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Size
You must remember you don't need a big image. Don't need very hight resolution because images print in screen, not in paper (in this cas, you need it). Thinsk about screen size and you get the answer to the question of "which is the best size?". If screens have 1200 or 1600px, you don't need more... Even, you usually don't use a image to cover all the screen, do you? Then with, for example, 800px by side, you usually get a proper image.
Ps: remeber, a lot of image viewers don't show images in a 1:1 scale. Check it when open files in your viewer...
Usually, web images have between 15-100kb. That's ok. A 600kb image is not suitable. You want your web load quickly!
Software
You can use the free, opensource and multiplatform Gimp. It's a wonderfull program, with similar features than photoshop (almost for 99% of users). It's a complex program, but the crop and resize options are easy. You can also covert to alfa (make transparent) a part of the image.
If you use linux an easy way to resize an image is using nautilus addons, for example. As simple as using a mouse right clic option when you are browsing your image directory :-)