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---Third Party Image Resizer---
<%# CustomCode.ImageResizer("ImageFieldName") %>?width=150
<%# CustomCode.ImageResizer(Eval("ImageFieldName")) %>?width=150&height=200
<%# CustomCode.ImageResizer(Eval("ImageFieldName")) %>?width=150&height=200&mode=crop
<%# CustomCode.ImageResizer(Eval("ImageFieldName")) %>?width=150&height=200&quality=10
<%# CustomCode.ImageResizer(Eval("ImageFieldName")) %>?width=150&height=200&bgcolor=000000

More example of the third party plugin:
http://imageresizing.net/docs/basics

---Built in Kentico Image Resizer---
//Code for Image field  - DataType - Media Selection
<%# CustomCode.ImageFunction(Eval("ImageFieldName")) %>&amp;width=150
<%# CustomCode.ImageFunction(Eval("ImageFieldName ")) %>&amp;height=150
<%# CustomCode.ImageFunction(Eval("ImageFieldName ")) %>&amp;maxsidesize=150


//Code for Image field  - DataType - File
<img src="~/getattachment/<%# Eval("Image") %>/<%# Eval("NodeAlias") %>?width=240"  />