Disappearing images from plugins after SSL on multistore configuration

Hey guys,

I've been looking for a solution over a week now and I can't find anywhere. Here's my problem:

I have a multistore configuration with SSL enforced for all the stores. There are two plugins that handle some images for across the websites. Those images are disappearing after some time and the error that appears in firebug is err_cert_common_name_invalid which is a common error when the image doesn't come as https resource from it.

I go to the Admin area, configure the widget, select the store in the multistore configuration and hit save again and voila the images are showing again. This thing is happening like 4, 5 times a week.

Is that anything that I can do on my plugin's code to enforce the images to come as https resources?

Appreciate for your time.

My code to retrieve the picture URL

        protected string GetPictureUrl(int pictureId)
        {
            string cacheKey = string.Format(ModelCacheEventConsumer.PICTURE_URL_MODEL_KEY, pictureId);
            return _cacheManager.Get(cacheKey, () =>
            {
                var url = _pictureService.GetPictureUrl(pictureId, showDefaultPicture: false);
                //little hack here. nulls aren't cacheable so set it to ""
                if (url == null)
                    url = "";

                return url;
            });
        }