A few suggestions that I think will make your life much simpler.

Choose an image that you want to use for the theme image in the album. Make sure that it's a full-sized original, not an image that you've already made smaller or cropped extensively. It should be at least 2000px wide, and wider is better.

Add that image to the top level of the project. Now exclude it by clicking the Exclude icon at the top, or by hitting the "minus" key. Drag that now-excluded image to the Theme image box at lower right. This will cause the skin to use that image for the header, but it won't be included in the gallery as a separate thumbnail and slide image. Adjust the crop mask in the Theme image box by dragging it up and down so that the important part of the image is selected.

Now head into the Tiger settings. First, go to Site > Hero > Design > Background image, and choose Album thumbnail (yeah, it's not a "thumbnail" at all, but that's what the setting is). This will cause Tiger to use that theme image for every page in the album. You can change your mind later, and use Folder thumbnail, but that means figuring out an image to use for every folder. A lot of effort for not much effect.

Now head for Advanced > Site admin > Images and uncheck High resolution theme image. Depending upon your source images, this option is just asking for trouble.