I'm following these plans for a rain barrel platform: http://www.diynetwork.com/how-to/outdoors/structures/how-to-build-a-rainbarrel-platform
There is a step "bolster the legs" where they suggest a horizontal piece of 2x4 across each leg. Why this configuration (good image here: 
) and not triangular braces in the corners?
Depending on how deeply you've set those posts, the braces you show may not be adding any significant amount of rigidity. Diagonal bracing would be better, but may be overkill.
My 5'-high platform for two rain barrels (with posts sunk 3' deep) has no braces on the thin ends, and has diagonals only to provide more support to the middle of the longer sides. And frankly those might not be needed since the structural framing of the top is 2x6'es on edge, with the long sides half-lapped into the 4x4 uprights and everything tied together with serious hardware (which I admit is installed in triplets.) Starting its third year, rock-solid despite carrying up to 1400 pounds of static load at times. (Plus 200 of dynamic load when I climb up to adjust things.)
Wood's strong stuff. Amateurs (including myself) are as likely to overdesign as underdesign. I looked at a number of designs on the web, applied my own experience as a volunteer framer, and settled for what seemed not-unreasonable overdesign. ("Anyone can build a bridge. You need engineering to build one that's strong enough at minimum cost." I applied rule of thumb and then threw money at the problem.)
I wouldn't trust it with a rain barrel. The braces interfere at the laps, which adds some rigidity, but I'd either use diagonal bracing or rely on diaphragm strength, e.g. 3/4" exterior plywood screwed not nailed to the posts.
[ edit/afterthought: You might also consider the possibility that, under strain, the braces shown could split lengthwise at the fastenings. With plywood or diagonal bracing that wouldn't be such a worry. (I admit it: I tend to overbuild). ]
