I'd start with a throw blanket and some pillows on the couch. Do you have a coffee table?

You could paint the tv stand, bookcase, or cabinet, or all three of them. That would change the room quite a bit. You'd just need to prime with Kilz/Zinsser primer, and paint with whatever color you like.

I'm personally not a fan of the pvc stickers, or adding artificial flowers/vines. You could get slipcovers for the couches, and/or add some pillows and throws.

I'd start by removing the open piece of IKEA furniture. I have another use for it though so don't put it too far away. I'd then switch the love seat and the art above and the TV and angle the taller cabinet that is currently to the right of the loveseat. Then move the sofa facing the the TV.
The floor plan I drew up below shows where I'd add more colorful art. The two on either side of the TV would be tall and thin. The one over the mantel would lean against the corner with a few decorative items on (tall) either side and something shorter in the front. I'd also add a floor lamp by the love seat with a colorful lamp shade. Place a couple colorful decorative items on the cabinet I placed in the corner. Pillows don't work too well on leather, they seem to slide and lay flat, so I'd go with adding a colorful throw over the sofa.
The reason I changed the layout of furniture is because the black seemed all to be lined up. Though I love the ceiling fixture, in my opinion, it's too feminine for the sofa and other furniture you have in this room, but if it stays, I agree some more feminine touches could help it fit in. I would not go with the stickers or vine you mentioned though. Instead, I'd add some live greenery. If you have enough light, add a plant on the half wall and maybe on the IKEA 2x2 that I put behind the sofa. You've a great start with the area rug pattern.

The short answer is that you cannot add little, decorative things to the room in hopes of changing it. The leather furniture and Ikea pieces are casual and clubroom-like. You would be better off expanding on that look. The overhead light in the room is too wimpy to fit in with the furniture. You are going in the wrong direction, IMO, with the idea of adding florals. In contrast, the big painting over the sofa is the right direction for the room - bold and uncluttered.

Lamps. You need lamps, especially a torchiere lamp.

Start by rearranging the furnishings to a more suitable arrangement for the room and add a larger more colorful rug. Then see what you might need to do. Do not bother with the decals.

Google decorate around black leather couch/sofa. I'd add a colorful rug and pillows. And some substantial art work.

Bev and justgottabme are on the same wavelength I am -- rearrange the furniture and get a rug that pulls in the colors in your art. I would try to get rid of the three small media pieces and look for a single larger entertainment piece or grouping. The smaller pieces (esp the smaller on and the TV stand) are too small for the scale of the leather pieces. Having three units creates a more cluttered look and feel when you want furnished and functional. They say the resale shops and craigslist see a lot of armoires and entertainment units -- go look for one that is anything but black and be willing to paint anything that is black or dark wood. If you paint, again, pull out one of the colors from the art. IKEA eould be another possibility.

If you look for an ottoman or coffee table, I'd look for metal and glass or color. Color can be paint or fabric -- a print or solid color ottoamn.

Replacing light fixtures is farily easy and need not be expensive, but I'd get your furniture placement and other things settled first so you know what height and other considerations need to be made. And add at least one or two lamps. Then you can decide whether you want toss pillows or a throw. For me, that kind of sofa doesn't call for toss pillows. A throw might be nice to have in the room.

After you rearrange, If you have blank walls, go to Marshalls, Home Goods, Pier 1, World Market, resale shops and look for mirrors, framed or wrapped art -- bring in some light metallics or repeat or compliment the colors in the larger art piece you already have.

Resist the temptation to try the decals. Florals and butterflies aren't going to play well with that leather. Your best bet is going to be looking for a modern, transitional to somewhat eclectic mix, even some MCM or arts and crafts -- those simpler lines to balance the heavier feel of your leather pieces but with enough strength and structure that they still hold up to them too. Does that make sense? It's almost go bold or go home -- and the butterflies and florals aren't bold. Tree trunks could provice a backdrop. A graphic that suggests wallpaper (saw a crysanthymum that was like a medallion repeat), a cityscape, -- there are possibilities -- just not the viney floral and butterfly.