Why is there no download option on Unique URLs viewed on mobile devices?

Due to the technical limitations of some mobile devices, this option has been disabled for all mobile sharing via Unique URL.

Who hosts the Unique URLs them and what costs are involved?

A unique URL (also known as a microsite) is a unique web address dedicated to your guest's shared content. LA Photo Party provides hosting of the unique URLs (at no additional cost to you) on photosfromyourevent.com.

It seems that the [[UNIQUE_URL_ENCODED]] isn't working on our emails but it's working on our unique URL page. Why is [[UNIQUE_URL_ENCODED]] not being replaced in email?

It may be that you do not have "Add share links to the footer of email:" selected in your event.  The tag [[UNIQUE_URL_ENCODED]] is only replaced in custom html template when the share links option is enabled, because it is used only in share buttons.

What is the recommended size for email and unique URL backgrounds?

Backgrounds of emails are usually 600 pixels wide. That is a rule of thumb used by many designers, but it is not absolutely necessary to follow.

There are a lot of factors to consider for Unique URL backgrounds. We typically recommend 1920 x 1080 so it will fit most modern screens. However, if you have to scroll down to see the full content of the page, you probably want to make it a bit taller. Alternatively, you could create your background so it tessellates and repeats seamlessly. Here's an example of a tessellating background:

If you're using custom HTML in your Unique URL, here's an article about recommended background sizes. It can be rather complicated!