Olov seems to have had the same issue here http://world.episerver.com/Code/Martin-Pickering/ImageResizingNet-integration-for-CMS75/

"I had to modify PostAuthorizeRequestStarted to get resizing in edit mode: 

protected virtual void PostAuthorizeRequestStarted(IHttpModule httpModule, HttpContext httpContext) 
{ 
var path = HttpUtility.UrlDecode(httpContext.Request.Url.AbsolutePath); 

if (path == null || !_isEditModeImageUrlRegex.IsMatch(path)) 
{ 
return; 
} 

Config.Current.Pipeline.PreRewritePath = _isEditModeImageUrlRegex.Match(path).Groups[1].Value; 
}"

Thanks, Petter!

You could also do like this when getting the url for the image.

@UrlResolver.Current.GetUrl(Model.ContentLink, null, new VirtualPathArguments {ContextMode = ContextMode.Default})?width=115&process=always&format=png"

Where the "new VirtualPathArguments {ContextMode = ContextMode.Default}" is the part were you get the ordinary url for the image instead of a internal episerver url.