haliphax

This StandardValidator extension will determine whether or not a given ImageField's source image's height and width match with given parameters. Either parameter can be omitted if, for instance, you only care about validating the width of an image (or conversely, only the height).

Note: In the code below, replace your.namespace.validators with something more suited to your particular project.

using System.Runtime.Serialization;
using Sitecore.Data.Validators;
using System;
using Sitecore.Data.Items;
using System.Text;

namespace your.namespace.validators
{
	[Serializable]
	public class ImageDimensionValidator : Sitecore.Data.Validators.StandardValidator
	{
		public ImageDimensionValidator() { }

		public ImageDimensionValidator(SerializationInfo info, StreamingContext
		context)
		: base(info, context)
		{ }

		public override string Name
		{
			get
			{
				return ("ImageDimensionValidator");
			}
		}

		protected override ValidatorResult GetMaxValidatorResult()
		{
			return (GetFailedResult(ValidatorResult.Error));
		}

		protected override ValidatorResult Evaluate()
		{
			// set initial return value
			ValidatorResult result = ValidatorResult.Valid;
			// grab the target ImageField
			Sitecore.Data.Fields.ImageField img = GetField();
			/* ignore blank fields
				* (use the Sitecore-supplied Required validator if you want blank fields to fail) */
			if(img.Value == "") return result;
			// use a StringBuilder for constructing the result Text
			StringBuilder txt = new StringBuilder("The image must be ");

			// are we checking width?
			if (Parameters.ContainsKey("w"))
				// mismatch
				if (img.Width != Parameters["w"])
				{
					// add to result Text and fail
					txt.Append(string.Format("{0} pixels wide", Parameters["w"]));
					result = ValidatorResult.Error;
				}

			// are we checking height?
			if (Parameters.ContainsKey("h"))
				// mismatch
				if (img.Height != Parameters["h"])
				{
					// already failed width? if so, add separator text
					if(result == ValidatorResult.Error) txt.Append(" and ");
					// add to result Text and fail
					txt.Append(string.Format("{0} pixels tall", Parameters["h"]));
					result = ValidatorResult.Error;
				}

			// failure - set Text to the StringBuilder value
			if (result == ValidatorResult.Error)
			{
				txt.Append(".");
				Text = txt.ToString();
			}

			return result;
		}
	}
}

Create a new field-level Validation Rule item in your sitecore tree (wherever you store your rules—in this particular project, they are stored in /sitecore/System/Settings/Validation Rules/Field Rules), and set the Type field for your new rule to:

your.namespace.validators.ImageDimensionValidator,your.namespace.validators

For the Parameters value, use h and/or w to pass through your desired image dimensions (i.e., w=320&h=240 will reject the image unless it is 320 pixels wide by 240 pixels tall).

Remember to swap the namespace with whatever you've chosen for your project.

Note: The version of Sitecore CMS this validator was built and tested on is 6.2.