"Paid, Permanent, and Plus users can create, edit, and delete custom mood themes with the [Custom Mood Theme Editor]. Mood themes for personal journals can be created and edited with [Admin Console] commands.
Images used in custom mood themes can either be [uploaded to your ScrapBook] or to a webspace provider that allows [remote loading to LiveJournal]. URLs for custom mood theme images cannot exceed 100 characters. In addition, you will need to input the width and the height of the image in pixels."
Brackets indicate links and the bolding is mine (added for emphasis). As I noted in a recent review, "the line about entering the dimensions is easily missed especially if you just read the first line, see the link to the custom mood creator and click that, and then you've navigated off the page and that nowhere on the creator does it mention that the dimensions are required - it might be helpful to include a reminder at the top of the theme creator."
I think that pretty much explains my suggestion: add a reminder or warning at the top of the custom mood theme creation page that says "Image dimensions are required. Not entering the dimensions of your image will result in the theme being created incorrectly." It may also be helpful to change the order of the FAQ so the line about inputting the dimensions is further up (maybe right after the first line?) and more forcefully worded: "You must input the dimensions of the image in order to create the theme properly." As it reads now, it seems obvious in retrospect that it's a requirement and not a suggestion, but I think it could easily be misconstrued.
It doesn't help that if you don't put in the dimensions, there's no error message of any sort to tell you that you haven't put them in; instead, it gladly creates the theme, removing all the images that don't have dimensions. I believe this goes in bugs instead of here, but I'll wait for someone to confirm that before I post a bug report.