Hi Patrick 67, welcome to the forum,
Did you modify existing icons. Or create new ones?
Are you talking about MS application file icons or folder icons?
I am using MS Paint (copied from XP OS and am using it in Win7) to create new icons (BMP) and then using a web based file converter to convert BMP to ICO files.
I find the MS-Paint (from XP) is better suited to pixel building than the Win7 version of paint.
But there are several icon apps on the web. I never bothered because they were commercial and my requirements were very minimal. I could not justify the cost.
BMP building is rather tedious but it does work.
What OS are you using? Snipping tool in Win7/8 is very handy for copying images as PNG then open in paint to save as BMP if you want to modify pixels. Then the web-based converter from BMP to ICO.
The web-based converter will also do PNG to ICO.
Let me know how you get on.
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NOTE my avatar was a snipped PNG file of an image which I touched up and changed colors in BMP format.