I'm trying to give title to a earth (no pluvial) water "puddle" of photo
It could be both a lacuna and a stagnum, both words are appropriate.
With this kind of question, a dictionary of synonyms will often help. These are dictionaries specialized in laying out the differences between words with similar meaning. A well known and online accessible one is Ludwig von Döderlein's, easily available online in the English translation by S. H. Arnold (1875). Here's what it has to say:
Lacuna denotes, in poetical language, any standing water, from a sea to a pool; lacus and stagnum are collections of standing water kept sound and fresh by their own springs, or by ebbing and flowing; lacus (liquere) is large enough to bring to mind the image of the open sea, in opp. to the main sea, like λίμνη; stagnum, like a pond, not so large as to resemble a lake, in opp. to a stream, like τέναγος; …
The entry also discusses palus, uligo, lama and lustrum, but these all seem inappropriate.
Forecellini fleshes out the difference between stagnum and lacus in his entry on the latter thus:
Differt a stagno, quod hoc temporalem habet aquam stagnantem, quae plerumque hieme colligi solet, aestate siccari: sed lacus aqua perpetua et perennis est.
So we can rule out lacus (which you didn't ask about anyway); the question is whether we can call this a stagnum if your garden pond is not “kept sound and fresh” by its own spring or fed by a river, and if it is artificial. I would say yes: for one, your pond is presumably kept sound and fresh somehow; Döderlein mostly appears to write this to delineate the opposition to paludes, uligines and similar things, which are corrupted, foul, thick and filthy. Also, the word can clearly be used for artificial ponds, e.g. Ovid (Pont 1, 8) reminiscing about stagna et euripi Virgineusque liquor of Rome (Virgineus liquor, in case you're wondering, is apparently water from the Aqua Virgo, one of the City's aqueducts).
Lacuna seems to be a very general word for any body of standing water, big or small, including ponds. I particularly like a quotation found in Forcellini, Palludius about keeping geese: Si desit fluvius, lacuna formetur (De re rustica 1, 30). Palludius is also clearly not a poet (nor does he count as a classical writer), but it does seem to be true that in this sense, lacuna is mostly a poetic usage. In sober prose I would prefer stagnum.