Looks like the sensor needs cleaning - it's one big weakness of dSLRs that when you swap lenses there is a fairly good chance that the sensor will attract dust, due to its relative charge. Also you mustn't brush or wipe the sensor because it will create even more problems - a friend of mine bought a "new" dSLR once and did the usual sky shot only to find lovely swirly patterns where the previous owner/user had tried to clean it himself
... needless to say that was returned for a refund. I am pretty sure he sends his cameras to be professionally cleaned every 6-12 months (there must be plenty of reputable companies out there) but I think you are very wise to avoid the high-street chains
