Either png or tiff will work to save your characters against a grey background. In PS, go to Layer -> New -> Layer from Background. Then go to Select -> Load Selection and pick the Alpha 0 layer and click invert. This will select the background color and you can simply delete it. Then use the move tool to place the figures on your background. Once you have all your figures placed in the scene, you can do things like shadow effects by copying a layer and putting its threshold value at the max. The transform tools can then be used to position the shadow on the floor. And then lower the opacity of the shadow so that it fits with the image. To add depth once you have all your shadows in place, merge everything down. Then create a duplicate layer and use the find edges filter. Change the effect of the layer from normal to soft light and reduce the opacity from 10 to 20%. Then create another duplicate layer from the original layer. In your droplets, select either old photo or oil paint. Then change the layer effect to soft light and reduce the opacity down to 30 to 60%. Merge all of your layers down. Hope this helps :)