I would immediately hang the background so it is "wide", not "tall". The issue with a narrow background is that it can be difficult to get the background to fill the width of a horizontal camera shot. Making the lighted background W_I_D_E_R will immediately give you a much easier set to work with, and will allow you to use different lens focal lengths with much more ease and fluidity. The lens focal length determines the angle of view behind the subject; a 24mm lens could fill the frame with the dog and sled, but the entire back wall would show! A 50mm to 100mm lens. ADDING more light by upping the flash power is the wrong approach, I think. Shutter speed and ISO are the easiest ways to regulate how bright the LED lights will be shown; the FLASH is regulated by the ISO used, and the f/stop, but the shutter speed has zero effect on the flash exposure. You can make the background lights as dim or as bright as you want, by changing the shutter's "open time duration"; adding more flash can actually be counterproductive. If you ADD flash lighting on the subject, it will tend to make the background LED's register as dimmer... Shutter speeds of from 1/40 to 1/8 second could be useful, for getting the LED lights to register at different brightness levels, when using say one speedlight, ISO 200, and f/8-ish. Thse LED's do NOT look all that bright to me, based on your f/2.8 exposure setting. Lens aperture size, in both f/stop (focal ratio number) AND the actual, physical width of the hole in the lens (which determines background blur, and also, depth of field. Ergo...with a small lens, like a 50mm lens at f/2.8 you get smallish bokeh balls; with a 200mm lens at f/2.8 (same focal ratio), the p_h_y_s_i_c_a_l_l_y w_i_d_e_r hole in the lens will create LARGER bokeh balls. So...if you want larger out of focus bokeh balls on the background LED's, you will want to use a larger f/stop with a physically longer lens. Moving the tripod as far back as you can, and using the absolute longest focal length lens you have (135mm,180mm,200mm,300mm) will make the background blur circles (the bokeh balls) BIGGER. This is physics...depth of field is discussed a lot, but not so the topic of background blur. The issue with longer focal lengths is that you can get the dog to be say 1/3 the width of the entire frame, but the longer the focal length used, the narrower the angle of view _behind_ the subject. So...it's always a balancing act....camera-to-subject distance, and focal length used, and the amount of physical background width shown in the picture. Wide-angles will show lots of backdrop width; extreme telephotos will show a narrow slice of the backdrop. ********** Looking at the basic exposure info you gave: ISO 200, f2.8 and the speedlight at 1/8 power. Whatever the shutter speed you used, it looks GOOD. There's absolutely nothing wrong with the speedlight at 1/8 power for f/2.8 and ISO 200 and your shutter speed: you have a VERY nice balance of the exposure triangle!!! But, you could get bigger bokeh balls with a longer lens, at f/2.8 or even f/4 or f/5.6, from much,much farther back than with the 50mm lens.