Sorry about the delay, but I think you've misunderstood me.
You need to resize the uploaded image to the size you want it to display (we'll call this full size, even though you might have resized it) in the slimbox, then when you insert it in your post you associate a thumbnail of an appropriate size as well.
Once you've saved your post, when you click on the thumbnail the slimbox comes up with the "full size" image. In this case, you made your full size image the same size as your thumbnail (350x178), so clicking the thumbnail displayed - that's right, a 350x178 image.
You want to save something larger (but not as overly large as the original pictures you'd posted), maybe something akin to 800x600 (adjusted to match the dimensions of your image), use that as your "full size" image, select the thumbnail size you want, and then it should do what you're anticipating.
As for a "blank white screen", I assume you mean a white overlay? When I brought up the image there was a white overlay, which is basically a layer of whatever color you selected in the options adjusted to whatever opacity you'd selected as well. If it's solid white, that means you set opacity to 1, but when I checked it, it was something lower than that.
Hope that helps.