I'm not sure if it's just the thickness of the "dimensional" mediums that make them dome or not, but if you're talking about clear mediums like Diamond Glaze and Crystal Lacquer, etc., those aren't resins (like Envirotex Lite, Easy Cast, or polyester resins). They're actually permanent white glues, tweaked a bit. They will dome even inside a "cell" and won't shrink back like resins will, but they are softer than resins and will need to have a coat of polyurethane/etc on top to keep scratchability down and also cloudiness from absorbing moisure over time.
(There's more info on those mediums on this page, if you're interested:http://www.glassattic.com/polymer/finishes.htm
...click on Dimensional Finishes
Epoxy resins will dome though too but they'll only do that (in one pass) if the resin is applied to a flat-surface object while it's elevated (and has no walls/etc to create a "cell"). The way that's often done is to set the flat item on a stack of pennies (as long as the diameter of the pennies is less than
the diameter of the item itself), pour the resin on (some people like to do that in a particular motion) until it domes from surface tension (any excess will fall off if you've used too much**), then let it cure. (Make sure the object or photo/etc is perfectly level though or the resin will end up thicker in one area than another--more important for larger surfaces.)
**so put some wax, Vaseline, or masking tape on the back side to resist the resin or some drips will end up there and perhaps have to be sanded off later if they'll matter.
Can't quickly find a good pic of that process but this video from TapPlastics shows the basic method on a much larger surface beginning at this point in their YouTube video:http://www.youtube.com/watch?v=oXK98Qohjjw#t=3m29s
(the doming is harder to see because the surface being covered is so large... it would be much more visible on a small photo or flat object like something used for jewelry)