HTML/CSS question re: z-index

What are the z-indices of images and HTML links that don’t have an explicit z-index? Specifically, what’s being attempted is to place a semi-transparent image over a portion of a page and retain “clickability” for the links thereïn.

If it’s relevant, the code for the links comes after the code for the semi-transparent image (due to the restrictions on what sort of code is allowed on the webspace available (a Neopets userlookup), this order can’t be changed and pretty much all coding must be CSS).

If a z-index is not explicitly assigned, the first object drawn on the page has the lowest z-index; the next item drawn comes next, etc.