there a certain conventions on some sites/competitions to either photograph minis always against a light blue background or inside a white light tent.
this makes them better comparable.
others do cut the image and pose them before scenary or other backgrounds. this most often doesn't look okay because the light direction or colour of light in the background differs from that of the mini...
what I personally do at the moment is having a pack of different colour paper sheets and trying to choose one that accompanies the mini. this can either be a colour that supports the colour of the mini (e.g. a brownish or greenish colour for Radagast) or makes a nice contrast (e.g. middleblue for heavy golden armoured mini).
the background shouldn't be too bright. otherwise it might get overexposed or the mini look too dark.
of course, taking minis in their more or less natural surrounding is even a better alternative. Radagast midst moss and roots in the forest, Gollum on a cobblestone in a puddle...