Image is an ambiguous reference between System.Web.UI.WebControls.Image and System.Drawing.Image

hi 

i use this link for resizing image

http://tuvianblog.com/2011/07/07/how-to-create-thumbnail-image-in-asp-netc-or-resize-the-image-before-upload-in-asp-netc/

i wrote code but it make error for these line

 using (var image = Image.FromStream(sourcePath))

CompositingQuality.HighQuality;
SmoothingMode.HighQuality;
InterpolationMode.HighQualityBicubic;

Error17The type or namespace name 'ArrayList' could not be found (are you missing a using directive or an assembly reference?)

i should add name space for them what should i add?

best regards

thanks