image conversion

Yes you can use websites as some people are recommending in their answers but if you learn how to do it right, you can probably do it yourself in the same amount of time it takes to browse to the site, upload the file, wait in the queue and download it again. The disadvantage of using the sites it that you usually can't control the sizes the or image quality of the individual sizes within the icon.

Here's how you do it manually. You can use GIMP which is an open source alternative to Photoshop that can download for from the project's website. No additional plugins will be necessary. Take the PNG you want to convert. It should be at least 128pixels in size but 256+ would be better.

Open said png in Gimp. Your going to make an image with several layers. When you finish each layer is going to be a different size in the finished icon. Here's the layer layout I usually use 256px (optional), 128px, 96px, 64px, 48px, 40px,32px and 24px. You want to have this diversity in sizes because you need to account for all the different zooms the user will have while browsing in explorer. 256 is overkill but why not future proof your icon?

Step 1: import png into gimp. I usually do this by using Open With in windows on the png and selecting gimp. Gimp will do the rest after that. make sure it's the only layer at the moment (delete any background layers if gimp made any).

Step 2: Resize picture to 256x256 (or 128x128 if you don't want to use 256) by clicking the Image menu and selecting Scale Image. If the image you're stating with isn't square, you'll have to crop it before doing the scaling or it will look weird. You can crop by using rectangle select tool (make sure you do a square selection by pressing shift while selecting). When you're done selecting, click on Image menu and select Crop to Selection.

Step 3: Duplicate current layer by selecting the Layer menu and selecting Duplicate Layer. Click Layer menu again and select Scale Layer and resize duplicated layer to new size like 128 or 96 etc.

Step 4: rinse and repeat until you have made your layers all the way to 24px.

step 5: export current drawing as an Icon by clicking on File menu and selecting Export As. When you get the dialog prompt, you want to make sure that you put in the correct extension (.Ico) and gimp will handle the rest.