1) You can wear a dress. You can wear whatever the heck you want.
2) If most of your weight is in your belly area, have you considered some sort of corset or girdle? That might help you look a little slimmer and when you look at pictures you don't feel bad. I wouldn't wear Spanx; Spanx are for skinny people with minor bulges.
3) Here are some dress styles for people whose weight is mostly located in the midsection:
Gowns that accentuate the smallest part of your torso (which is right under your boobs), and then flares from there to the floor.
Gowns that have a rouched waistline. The rouching conceals our conflict area:
Wide V-necks like the ones I showed you above are great because they create the illusion of a thinner waist.
And then here are some ideas/option if you don't mind wearing a girdle/corset:
The trick is creating the illusion of a longer, slimmer torso through rouching in the right places, a V neck, and flowy fabric. You can also go for a structured, tailored dress as well. I carry my weight in what I call the 4Bs: Back, Belly, Boobs and Booty, so I have kind of learned to play around with styles to conceal my belly and make my figure a bit more stylized (also because I'm 5'4'' and being over 200lbs made me look pretty big).
I hope this helps!